Like the others have said, so long as you also have the rank of "Guild Leader" in the guild menu (that purple crown thingy irrc) then you SHOULD be able to demote him to some other rank, and/or restrict his bank access (through rank restriction) so he can't do anything. I'm not 100% sure if you can demote the person who made the guild though... but, I do know a guild can have more then 1 person at the "leader" rank, so you should be able to just demote him if you are also the same rank as him.
If that doesn't work, try talking to him about it. Assuming he is willing, he can promote you to leader rank (if you aren't already) then leave the guild/be kicked. When he comes back in it SHOULD be at the lowest rank, effectively making you the only "leader" rank if you can't demote him.
Barring either of those, and again if he is willing, you may have to both call a GM (well, maybe one of you while the other will be on for awhile... both may be over-kill) and talk to him to confirm the story though.
If that doesn't work (aka the GM wont do anything about changing the guild leadership around) that was basically the last thing I could think of, so you're basically SOL and either have to live with him as a "leader" (with you having the same privalages as he of course, if he promotes you) or make a new guild with those loyal to you and dislike him.
Oh, and do try these things in order... the last one with the GM is basically the extreme end of possibilities and is VERY last-ditch.