Your players now have the ability to throw out massive fireballs, walls of fire and ice storms so what? I know there are many DMs out there that have no problems with this but no matter what do not nerf your player's abilities as they get more powerful. Sure there was an antimagic field in the last game but notice that they were embedded within the oytugh creatures and although were necessary to get into the room they were not necessary to defeat the beastie. Good DMs will tell you that if your players get new powers and abilities and you nerf them right off the bat without explanation or a story-reason you may end up with angry players.
Best quote of last Saturday night's game was by my wife "Is there a protection from death spell?"