It's already a rule. It's harder to spot unless we watch who draws it, but we treat it the same way as porn sprays.
If every rule was stated in the motd new players would not bother to read it. After my exam today I can start to compile a list of the unwritten rules for future refrence.
On the Internet you can be racist, sexist, or just generally a dick, and be accepted; but god help you if you have bad grammar.