In 1985 Law 26 is 12 pages long and covers many different scenarios.
The relevant bit is:
[LAWS]Misconduct, Dangerous Play
(3) It is illegal for any player:-
(a) to strike an opponent;
(b) wilfully or recklessly to hack or kick an opponent, or trip him with the foot, or to trample on an opponent lying on the ground;
[...]
Note (vi)
A player shall not "take the law into his own hands" nor wilfully do anything that is dangerous to an opponent even if the latter is infringing the Laws.[/LAWS]
