When posting as a Guest, the forums prompt the user for a confirmation code as stated:

Enter the code exactly as it appears. All letters are case insensitive, there is no zero.

The wording of that is not correct with the comma since there is no "and" given. As well, it may be possible that more than one "O" may appear, which could be interpreted as a "0" (zero). So to assume that more than one "O" may appear, it would be best to say "zeros", and not simply "zero".

Instead of saying "All letters are case insensitive", which is fine, it may be less confusing for some users to say "Letters are not case-sensitive". That is a matter of personal preference.

So as a final return, I think this would be more appropriate:

Enter the code exactly as it appears. Letters are not case-sensitive, and there are no zeros.