Do you want the Enemy to stop forever, or just pause, and get back to moving? Do you want it keep going but to bounce off at an angle like a billards ball?
(In my examples I have a TiledBackground called Wall)
Enemy: On collision with Wall=> Enemy: Set bullet disabled
Stop and pause:
Enemy: On collision with Wall=> Enemy: Set bullet speed to 0
No events needed, just make sure the Wall has Solid behavior and the Enemy has its Bullet behavior setting to Bounce of solids set to yes.
Hope this helps, if not, give me more details on what you want to accomplish!