How can I make a Django form field contain only alphanumeric characters

You would use a validator to limit what the field accepts. A RegexValidator would do the trick here:

from django.core.validators import RegexValidator

alphanumeric = RegexValidator(r'^[0-9a-zA-Z]*$', 'Only alphanumeric characters are allowed.')

name = models.CharField(max_length=50, blank=True, null=True, validators=[alphanumeric])
email = models.EmailField(max_length=50, unique=True, validators=[alphanumeric])

Note that there already is a validate_email validator that'll validate email addresses for you; the alphanumeric validator above will not allow for valid email addresses.

Instead of RegexValidator, give validation in forms attributes only like...

        class StaffDetailsForm(forms.ModelForm):
             first_name = forms.CharField(required=True,widget=forms.TextInput(attrs={'class':'form-control' , 'autocomplete': 'off','pattern':'[A-Za-z ]+', 'title':'Enter Characters Only '}))

and so on...

Else you will have to handle the error in views. It worked for me try this simple method... This will allow users to enter only Alphabets and Spaces only