Set a delete trait on all pieces. (probably you have done this already)
On Main Map, trigger a key command on all pieces on movement ending. ex. Ctrl-E
On all pieces add traits that do the following (use prototypes):
Trigger action that is triggered from the movement ending (Ctrl-E above) and does two things.
First is to write the piece name to a global property ex. testGP .
Second is to trigger a Global Key Command trait (with restricted and fixed range of 1). That will activate a new trigger action trait on all pieces in the same location.
Finally, set this new trigger to activate the delete trait of the piece if the $BasicName$!=$testGP$
You are done!