How to tell a technician I don't want him working on my viola?

Hey Viola gang,

I rarely post but I wanted to get actual violists opinions on this. The luthier shop I've been going to the past 7/8 years has some great technicians who actually play viola/violin, but I am consistently getting stuck (sorry, assigned I guess?) with the one technician who doesn't play viola. He says he is a string player, but he plays a zither, which is not the same AT ALL.

He's a really nice guy, very sweet and remembers me, but I am disappointed and frustrated by his work every time. It's always less resonant, and more nasal after he touches it. I go back, ask for someone else, have to wait a week or two, and it's just a huge pain in the ass to make at least three trips to the shop to get the sound correct after he touches it.

So my question is- how do I tell this guy and my shop that I don't want him working on my instrument in a kind way?