Played in a casual game just a few minutes ago and ended up on the same team as a cheater with a Medic bot on him. When I tried to votekick him it kept saying "You cannot currently kick this player." in the pop-up window or something similar. The same thing happened when the others in the team tried to kick him.
You could vote against the Medic bot but even if the majority vote yes he still wouldn't get kicked somehow.
I'm just making an assumption that it's a cheat, I don't really know anything about cheats nor how this could be set up, but if anyone has a clue would be interesting to see how that works.
Here is the Steam acc of the cheater:
https://steamcommunity.com/id/VAC-Is-Only-Spying
Honestly if Valve doesn't do something about this I feel like in-game casual could just be dead cause as soon as a cheater would join, you just wouldn't be able to kick them at all.