There are two huge schools of thought on this and you sort of have to gauge your cast. I have always dealt with this with a grain of salt, and when confronted by someone pulling pranks I pretty much give the following speech “I understand that you are getting bored playing your role, but although this maybe you 50th time (100 th time, 200th time, 1000th time) playing this role, it’s the audience’s first time and they are paying quite a bit of money. If you feel you need to do something back stage to help keep your performance fresh and exciting, then by all means go ahead – here’s the thing . . . it can not affect the product at all and it can not affect an unwilling participant.”
I worked recently with an amazing actor, big Broadway, West End type . . . he was great with little pranks . . . but only SM and he knew about it – like what sort of silly prop was in his pocket or hand. But it never affected the final production, it only gave him the rush of knowing something was afoot.
Now, you say small pranks lead to bigger pranks. Well, it can be that way . . . but in my history, small pranks release the pressure. When you come down hard with the "Zero Prank" mentality, they begin to become more covert – and wouldn’t you rather be in on the pranks and able to put your foot down and tell them what is acceptable or not, then kept in the dark?
Let the pressure be released in an acceptable manner - keep the energy and performance up. Remember, that can be some fun in theater, as long as it never makes it on stage, and the final product is not affected.