My left crank arm keeps coming loose on the square taper BB -I can't tell for sure but whats most likely at fault the arm or the bottom bracket wearing?When I bang it back on and tighten it's fine for a while.I've tried not to ride far when its loose to avoid more damage.
The crank taper surfaces are deformed and the arm needs to be replaced. There is no other fix. Usually the bottom bracket spindle is fine in these cases, but it's not an absolute given and should be looked at. The problem typically originates from the crank being under-torqued initially.
Left replacement cranks are available and not expensive. You need to get the correct length, profile, hole orientation, and color permutation. There are several questions here about this.