How do I destroy only part of a tilemap?

I'm working on a small game where the object of the game is to fire a bullet over a (tilemap) wall and hit an object on the other side. but if the bullet collides with a certain point on the wall then id like only that block of the (tilemap) wall to be destroyed.

https://mega.nz/#!mZNR2ZCQ!quWlnQUOfc26 ... tztARNxVuQ

Thank you Fidasx. ive only just got around to trying your tip out.. it works nicely for me...im very greatfull that you took the time to help out. thanks again.

np anytime