I've just dug out the Malicious communications act and sticking it here, so I don't have to research it again when drafting forum rules.
Malicious communications
Section 1(1) of the Malicious Communications Act 1988 provides that an offence is committed by any person who sends to another person, with the intention of causing distress or anxiety:
an electronic communication which conveys a message which is indecent or grossly offensive, a threat, or information which is false and known or believed to be false by the sender; or
any electronic communication which is, in whole or part, of an indecent or grossly offensive nature.
Electronic communication is specified to include any communication sent by an electronic communications network, and any communication (however sent) that is in electronic form, and should therefore cover forum and message board posts.
Section 127 of the Communications Act 2003 provides for two further offences, very similar to that under the 1988 Act.
First, a person is guilty of an offence if he sends by means of a public electronic communications network a message or other matter that is grossly offensive or of an indecent, obscene or menacing character, or causes any such message or matter to be so sent.
Second, a person is guilty of an offence if, for the purpose of causing annoyance, inconvenience or needless anxiety to another, he: (a) sends by means of a public electronic communications network, a message that he knows to be false, (b) causes such a message to be sent; or (c) persistently makes use of a public electronic communications network.
Punishment under the 1988 Act or the 2003 Act may, again, be by way of a fine and/or imprisonment.