In all seriousness, the best way I've ever found to counter some player's consistently amazing/extravagant but legal behavior is to make them deal with it.
In this case, make an uber-infiltrator, and create a reason for him to be hunting the team and especially Mr. Stealthy. Let the team learn of the hunt, make a reason they get to learn just how effective this guy is, then let it run.
As a worst-case example: enemy sends message - "I'm the best. You're encroaching. I'm going to prove I'm better by interfering with you, then in your humiliation I'm going to kill you. And I'm so good you can't do a thing about it." (Basically a variation on the gunfighter problem.)