The thumb safety blocks the sear from moving inside when it is engaged in the SAFE position. That's what that little block extension on the inside surface of the safety is for.
At the same time as it does that, that block is in a position to halt the full rotation of the hammer as well. It is placed into the machined out arc in the lower part of the hammer that presents itself when the hammer is full cock.
This is another safety in that it blocks full rotation of the hammer should the full cock notch break and the hammer fall. The sear nose , still blocked in position by the thumb safety, is also expected to catch the hammer in the 1/2cock notch.
That small block on the back of the safety that does all this work should be a slip fit up against the sear ..
If not fitted correctly, it will jam itself into position betw those surfaces. There is very little leverage to pull it free. It wasn't designed to need any, the spring/plunger/detent on the outside of the frame is the 'hold it in place' mechanism for the Safety either ON or OFF.
If it does stick in position again, sometimes pulling the hammer back a small amount and holding it there while at the same time trying to release the Safety can help undo it.
If it's simply the plunger & spring causing the problem,,a less than rounded plunger end and/or the same less than perfect detent hole in the Safety,,then those pieces can be refit quite easily as well.
Added.
Here's a good diagram of the 1911 Thumb Safety block. Explains it better than I can.
The red area is the block on the back side of the thumb safety that does all the work. Here engaged On Safe blocking the sear and in position to block the hammer from full rotation should the FC notch fail.