Make portion of UITextView undeletable

for a complete solution you need to handle several cases, including the cut and paste operations that may start in the uneditable part and extend into the part which the user can edit. I added a variable to control whether or not an operation that includes the uneditable part but extends into the editable part, is valid or not. If valid, the range is adjusted to only affect the editable part.

// if a nil is returned, the change is NOT allowed
- (NSString *)allowChangesToTextView:(UITextView *)textView inRange:(NSRange)changeRange withReplacementText:(NSString *)text
                       immutableUpTo:(NSInteger)lastReadOnlyChar adjustRangeForEdits:(BOOL)adjustRangeForEdits;
{
    NSString *resultString = @"";

    NSString *currentText = textView.text;
    NSInteger textLength = [currentText length];

    // if trying to edit the first part, possibly prevent it.
    if (changeRange.location <= lastReadOnlyChar)
    {
        // handle typing or backspace in protected range.
        if (changeRange.length <= 1)
        {
            return nil;
        }

        // handle all edits solely in protected range
        if ( (changeRange.location + changeRange.length) <= lastReadOnlyChar)
        {
            return nil;
        }

        // if the user wants to completely prevent edits that extend into the
        // read only substring, return no
        if (!adjustRangeForEdits)
        {
            return nil;
        }

        // the range includes read only part but extends into editable part.
        // adjust the range so that it does not include the read only portion.

        NSInteger prevLastChar = changeRange.location + changeRange.length - 1;
        NSRange newRange =  NSMakeRange(lastReadOnlyChar +  1, prevLastChar - (lastReadOnlyChar +  1) + 1);

        resultString = [textView.text stringByReplacingCharactersInRange:newRange withString:text];

        return resultString;
    }

    // the range does not include the immutable part.  Make the change and return the string
    resultString = [currentText stringByReplacingCharactersInRange:changeRange withString:text];
    return resultString;
}

and this is how it gets called from the text view delegate method:

- (BOOL)textView:(UITextView *)textView shouldChangeTextInRange:(NSRange)range replacementText:(NSString *)text
{
    // did the user press enter?
    if ([text isEqualToString:@"\n"])
    {
        [textView resignFirstResponder];
        return NO;
    }

    NSInteger endOfReadOnlyText = [self.spotTextLastSet length] - 1;
    NSString *newText = [self allowChangesToTextView:textView inRange:range withReplacementText:text
                                       immutableUpTo:endOfReadOnlyText adjustRangeForEdits:YES];

    if (newText == nil)
    {
        // do not allow!
        [TipScreen showTipTitle:@"Information" message:@"The first part of the text is not editable.  Please add your comments at the end."
                      ForScreen:@"editWarning"];
        return NO;
    }

    // lets handle the edits ourselves since we got the result string.
    textView.scrollEnabled = NO;
    textView.text = newText;


    // move the cursor to change range start + length of replacement text
    NSInteger newCursorPos = range.location + [text length];
    textView.selectedRange = NSMakeRange(newCursorPos, 0);
    textView.scrollEnabled = YES;

    return NO;
}

sch's last edit makes a decent answer, but I want to offer a slightly more flexible approach.

You have to keep in mind the copy/paste system. User might select all the text in text field and try to paste in the entire value which might be perfectly acceptable, but if (range.location <= 9) { return NO; } will reject it. The way I'd do it is put together a string that would be a result of successful edit and then check if that string would start with your desired prefix.

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    NSString *resultString = [textField.text stringByReplacingCharactersInRange:range withString:string];
    NSLog(@"resulting string would be: %@", resultString);
    NSString *prefixString = @"blabla";
    NSRange prefixStringRange = [resultString rangeOfString:prefixString];
    if (prefixStringRange.location == 0) {
        // prefix found at the beginning of result string
        return YES;
    }
    return NO;
}

Edit: if you want to check if the current string in text field starts with the prefix, you can use rangeOfString: the same way:

NSRange prefixRange = [textField.text rangeOfString:prefixString];
if (prefixRange.location == 0) {
    // prefix found at the beginning of text field
}

You can use the delegate method textView:shouldChangeTextInRange:replacementText: To tell the text view whether to accept the delete or not.

As the documentation says:

range : The current selection range. If the length of the range is 0, range reflects the current insertion point. If the user presses the Delete key, the length of the range is 1 and an empty string object replaces that single character.

Edit

Here is an implementation where the user can't delete the the first ten characters. But he will be able to insert characters there.

- (BOOL)textView:(UITextView *)textView shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    if (range.length==1 && string.length == 0) {
        // Deleting text
        if (range.location <= 9) {
            return NO;
        }
    }
    return YES;
}

Here is an implementation where he can't modify the first ten characters at all.

- (BOOL)textView:(UITextView *)textView shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    if (range.location <= 9) {
        return NO;
    }
    return YES;
}