Try a can of "pipe freezer", make sure you only get it on the alli action block to shrink it, but alli shrinks much more than steel for a given temp drop so even though the cylinder will cool the grip on the block thread should relax.
If you want to use heat you will only need boiling water to free any thread locking agent or paint in the thread. Quite easy to stick the trigger block end in a pan on the stove for 10 mins, this may free it and it shouldn't take the paint off the block, however it will soften it so be careful if you have to clamp it while hot (although the cross hole with a bar is the usual method).