Which assumes that you have the mind control unit in a MyUnit Unit variable, and you are using a Generic expiration timer. Im not sure if this works, but its worth a shot.
I've already tried removing 'BTLF', but it kills the unit instantly. I tried pausing the timer, and then removing the ability, but it neither worked. The only thing that you can do is replacing the unit, but be carefull, all variables that refer to the original unit will be 'nullified'. Problems could occur with items, and aahh yes, abilities (if we're talking about a hero).