I don't play gungame much anymore, but I use to be quite regular. I've been around since the first iteration of the mod and even ran a pretty decent server for a while. This has been around since stealing levels became possible. I don't see what you could do about it, nor do I think a complaint is needed. To whether its game ruining or not, its perspective. Obviously the other team likes the challenge of knifing people who are shooting at them (which gun game rewards). You're talking about changing the entire gun game dynamic IMO

Either buck up, aim better, or pull out your knife :-P