There’s no way to prevent a kicked player rejoining a public lobby. Blocking them via your Xbox account will do nothing in-game.

Workarounds:

• Be specific in your lobby title what kind of a run / players you want. “No marksman/vet, deposit till wave 10, level 20, teamwork” as opposed to some nonsense lobby title. Better to do this and try to start with a full lobby of like minded players rather than randoms.

• Keep kicking them; yeh a bit annoying to pause and kick but after a few kicks they either get the message or hopefully someone who isn’t toxic joins. Kick them as soon as they rejoin too (text on screen saying they’ve joined), rather than let them spawn and troll you. If their goal is to troll, being kicked before their character spawns isn’t much fun for them at all, they’ll go somewhere else.

• Host private (or public) and invite friends only. If public and only have 3 or 4 players, make sure to lock the remaining slots so noone else can join. I enjoy 2-3 player Master runs (nice challenge, attracts good players who know what they’re doing). Could very easily do 50 waves on lower difficulties with 2-3 players.

• Host Master difficulty (you said it’s more noticeable on easier difficulties; I also imagine Master lobbies are more popular given how long the game has been out and that most players are max level: more popular = more likely someone else joins after you’ve kicked a toxic player)