Spawn trapping like the kind found in Dome Sabotage and Hardhat CTF should be bannable.
The enemy can't even react, you can knife them as they spawn. A lot of the Sabo spawn trappers have friends join their session to make setting up the trap easier.
Blame the devs, not the players
While it is bad sportsmanship, it's still a legit strat as it involves no modding or cheating whatsoever. It's kind of like something in this other game called Command & Conquer 4: Tiberium Twilight, where you can use something called a Stasis Shield to stun all your enemy forces, essentially locking them out of the match. It's not nice, but still technically legal and, therefore not ban-worthy.