There have been a number of threads reporting this feature doesn't work. I don't know how Paradox intended it to work, but I have a suggestion assuming this will be eventually fixed. Have forced disarmament empty the mobilization pool. This will reflect the confiscation of weapon stockpiles, reassignment of NCO's, and general interruption of the reserve system. The disarmed country could immediately begin building the pool up again, but if it had say a 100 division pool prior to being disarmed it will have to start from scratch. In this way you'd be able to see tangible results and not wonder whether the disarm option was a wasted action.