There may be something stuck between the blades. Turn the mower over and look for an obstruction. If you don't see any, you may have to oil the blades to see if that frees it up.
I do believe that it may be time to turn your riding lawnmower over and check the blades themselves. Take the blades off of the lawn mower, and check them individually for problems.
There are several things that could cause your mower to not disengage blades. Check the belt size, make sure there is no rust in the pulleys and that none of them are bent.