Getting purposely tagged, unless constantly done (and by then they should be banned for that) will not create that much rank manipulation so unless you constantly allow someone to break the rules, it should be the last thing you ban them for. One of the only actual cases of rank manipulation ever was Hallwagner (he went from 200-top10 in about a week) and he would spawn zombie every round on ze's and tag upwards of 5-10 humans a round, hence padding his stats. Most people only can tag 1-2 people by doing that in a zm barricade, which a good zombie will beat even if they have to break down cades.
"To attempt to have intercourse with a hornet's nest is a very bad idea," Siv During Livh, a psychologist and expert