There are no written rules but it's all a matter of good manners really. If a DJ discovered, or is associated with, a particular record, you ask if he intends to play it before you decide to.
Not sure this applies to Johnny Howard though, (far more energetic version than the Artistics BTW), because as Pete said it was played in the seventies, and there must be plenty of copies about.
There's no excuse though for removing someone else's record from the decks or any other "militant action".
From what I've seen recently, some of those who are the most sensitive about being able to play "their" records have no such respect for records first played by others.
As for the guys playing Mello Souls, Primers, Margaret Little boots/dubs, the promoters know what's going on and they are as much to blame as the DJ's IMHO.