Dimensions

I created a dimension where a small explosion goes off when you enter it, destroying the portal and stranding you there until you can build a new one. The problem is that I did this with elements and the explosion also goes off whenever I re-log into the world, it's really annoying. Is there any way to fix this? On a similar topic, I'm also having trouble making it so only my custom mobs spawn. Despite there being no grass, mobs like chickens and sheep still spawn all over the place. Is there any way to prevent this? I would also like to make it so other hostile mobs like zombies and skeletons don't spawn if possible, it's clearly bright in the dimension so i don't know why they're there.