How can you disable the internal speakers with a hardware solution?

You may also need a new set of headphones as the connector just don't fit quite right into the socket.

Look at an Apple set and look at yours note the total length as well as the detent (the thinner point just behind the tip). Some are slightly short or the detent shape or location is not quite right.

The socket has a switch which as others have pointed out does get damaged. If I remember correctly the switch closes to enable the internal speakers. In your case you may want to pull the logic bd out and solder a wire across the socket switch so the headphone socket is always active. Otherwise you'll need to replace out the logic board to get the socket working. If you can find a dead system for parts you might be able to swap out the socket.

With it jumping back to the internal speaker, i believe there is a bad connection on the audio jack, either where it connects to the headphones, or where it is soldered on the board (or flex cable, however it is attached). Especially if you can wiggle it. Also with wanting to remove the built in speaker functionality, why dont you just remove the speakers all together. Desolder them? Should be fairly cut and dry....

With some of these units and past versions of the MBP, if you carefully use a paper clip stretched open and put it in the headphone jack and carefully use it to manually turn the part which senses if the sound is input or not it will sometimes clear the issue. It may have to be done more than once or it may not work but occasionally it does solve the problem.