I am assuming that you are talking about 1 on 1 situations and random PVP and not about OP figths since "Not every class has a way to fight it so no this is not a viable solution in my mind." is no valid argument during OP fights. Plasma weapons for example are able to hit people while stealthed. Then there are AOE weapons, winding argument comes to mind, which can "mark" spies and make them visible for everybody. And of course there is the true sight sanctum, USE IT!
As for 1 on 1 situations ...
I fail to see how these radiation stacks would enable, say a rifle PE (any class really) to kill a spy in 1 on 1 situations. I believe all these radiation stacks would achieve is discourage PVP, because spies would simply stealth sooner than they do now or they just stealth and run away.
I get the feeling people are upset about the spies healing up during stealth, but you should keep in mind that spies cant use nanites during stealth and usually you dont have the time to inject yourself before stealthing when you are in the process of being killed. Even if nanites are still running while stealthing their duration is very short. All this means is that the person not stealthing can heal up even better than the spy.
You could simply disable health regeneration during stealth, but again this would discourage PVP.