I think Slashdriver has the right idea. Find out if it matters at your track. Do some back to back testing with fresh but broken in tires and worn tires and see if there is a performance difference. If there isn't one or if the worn tires perform worse then there isn't any reason to police tires. Unless the worn tires give an advantage there isn't any reason to cause issues and tell people to get rid of them. At some point the risk of having a tire separate on the track is higher than putting on new tires.
If running worn tires does give a performance advantage then spec some rules -- tread visible the entire width and circumfrence for example -- and then decide when to tech for it -- beginning, random time, end, etc.