You've basicly got 2 options.
1: Keep altering strat/equipment untill you can actually beat him.
2: Try to move on to primus as soon as possible in an attempt to avoid him.

If your glad is in the top 10 of his/her bracket that means they can get challenged for as much as another gladiator likes. If you can win consistently against a gladiator than the easy way to get wins is to keep challenging said gladiator.

It's a scummy way of doing things but I cant disapprove completely as I've done it a couple of times in the past myself

Another way would be if somebody that's in range of the glad that's challenging you feels nice and starts challenging it however that will probably not stop the challenges but instead merely teach him a lesson of how it feels to be on the receiving end.