Are you talking about Tamiya's spray primer or the Liquid Surface Primer in the little 40ml jars?
If it's the LSP, you can strip it or clean it up with methylated spirits (ethyl alcohol).
If it's the spray can stuff, I'm afraid I have no idea. You might try Tamiya's lacquer thinner or Gunze's Mr Color tjinner. Both are "lacquer" thinners but are formulated to work with styrene models without turning them into molten puddles.