Got to agree with this comment, I would rectify the problem and make sure all future contracts are covered by tour T & C's and then just let these run there course.
As stated above 1000's of DJs do not even use contracts and there are no problems, why chance losing a booking by highlighting something that the only person that is worrying about is you.
In 5 years of using contracts I have never had to enforce my T & C's once and that is a lot more than 40 discos.
Chill out Charlie I am sure they will all be okay