The correct solution would be to cut the banana plug off the wire, leaving bare wire and leave the mounting bracket as is. You wouldn't want to have them come crashing down,now would you? But your idea of shortening the bracket should be fine as long as you drill a hole at the new bottom and get the bracket anchored really well. I would double check to make sure your plugs and wire don't stick out farther than the bracket does once installed on the speaker, before you modify the bracket, as it is easier to snip and strip the wire.
