Murdering the Yeast?

Coming up, I'm going to brew an 8 gallon batch of Dunkelweiss. Five gallons will be "real" Dunkelweiss, fermented with Wy3068. The other three gallons will be fermented with US-05 and secondaried with some cherry puree.

Last time I made a cherry wheat, the yeast fermented the cherries resulting in a "wine" flavor. Which I did NOT like.

What's the best way to kill the yeast in the secondary? Isn't there something called "Campden Tablets" that kill off yeast and bacteria?

I've never used it. Does it effect the flavor?

So long as you aren't planning to carbonate naturally, yes. Campden or Pottasium Sorbate should do nicely with no contirbution to flavor.

Back flavor, and keg to carb.

I personally don't like the taste of sorbate, but in small quantities, it's barely detectable. Use one campden tablet per gallon, and 1/2 teaspoon of sorbate per gallon, dissolved in some boiling water (a tiny bit- like 1/4 cup). Wait a few days, and then you can rack onto the cherries.