It's not a set rule that can be reinforced but their riders can be asked not to by the management and decide if they want to or not, although I imagine if it affects their own chances of making the play offs then they won't.
The risk they then run though is if they need guests themselves.
Coventry have been very good even with play off rivals. They have let riders guest for Belle Vue, Poole and a lot for Swindon.