Are you 110% sure that there is no pain issue? Her saddle fits perfectly (checked by a qualified fitter), her bit or bridle isn't pinching? You're not accidentally thumping down on her back when you put your leg on?
All of these aside, I would then be sending her to an experienced rider, who can sit out bucks and teach the horse that bucking is a bad idea. You don't sound to be overly experienced, and allowing the horse to work with a 'uh uh' is teaching the horse that bucking is the correct reaction to a leg aid. Without meaning to, you have actually trained this horse to buck on cue.
Send her to someone experienced, that will get on, put their leg on, and if she bucks, keep the leg on and either take her head off immediately, or give her a good whack on the backside with a dressage whip. Repeat until she realises that bucking is worse for her than the rider, and going forward is a heck of a lot easier.