generic NOT constraint where T : !IEnumerable

No - there's no such concept either in C# or in the CLR.

I found my self trying to implement the same case mentioned in the comments:

void doIt<T>(IEnumerable<T> what) { }
void doIt<T>(T whats) { }

I excepted the following code to reference the first method:

doIt(new List<T>());

But it actually references the second one.

One solution is to cast the argument like this:

doIt(new List<T>().AsEnumerable<T>());

The cast could be hidden by another overload:

void doIt<T>(List<T> whats) {
    doIt(whats.AsEnumerable<T>());
}

As far as I know it is not possible to do that.

What you can do is some runtime checking:

public bool MyGenericMethod<T>()
{
    // if (T is IEnumerable) // don't do this

    if (typeof(T).GetInterface("IEnumerable") == null)
        return false;

    // ...

    return true;
}