The Thompsons (like most SMGs) fire either semi or full from an open bolt, which is held back by the sear. If the sear does not hold, the gun will go full auto regardless of the switch position. It is also possible that the disconnector is broken or not working properly. (In the Thompson, the disconnector is in the top front of the trigger group.)
Assuming there is no dirt or anything interfering with the proper operation, I would try replacing the sear. That is more likely to be the cause than the disconnector, but replacing both would be a good idea and should solve the problem.
Gun Parts Corp. has parts for the 28.