Just add more layers of stone blocks to make the wall thicker.
...Or, just put your stone wall a bit further away, so that Mario looses his super launch speed before crashing into the wall. He'll be stopped by it then.
TIP: I found out something cool using that pipe shoot effect.
Check this out.
He enteres the little "cage" when being shot out of the pipe. The first wall he goes through but the other one stops him. It depends on his speed, as he slows down a little after a half second, hes not fast enough to go through walls. The weird thing inside the Stone block cage is actaually a repainted vine.