So, I was wondering. I just used another ammo type, and I am wondering how to switch back to regular ammo, if that's possible at all.

Bump?

I"ve been wondering this myself. I think you just have to alternate between all the ammos.

You can't turn ammo off, if you really want to swap your weapon at a weapon locker then swap it back.
 
All the ammo are 'buffs' they cost you nothing so there's never a drawback to having special ammo on. Thus no way to turn them off.

You can turn them off, what are you guys talking about? Just press the button that it's mapped to again and it'll go away.

If you "deselect" your current special ammo, then you are firing regular ammo. The icon on your gun will change to reflect this.

Aren't the special ammos better than regular ammo?  Is there any reason to not shoot special ammo?

Yeah, anyway as people above as head, click the ammo type button again, and it'll go away. You'll have to recharge your powers again tho

I'm lead gameplay designer on ME2 you can't turn them off :( Seriously.