r_uifullscreen is a stock cvar
Values: 1 (enabled) / 0 (not enabled)
/seta r_uifullscreen 0
If it’s set to 1 and the server where you playing is restricting it, Punkbuster kicks you!
But beeing set to 1 doesn’t mean anything as cheat or attempting for cheating, so by default it can be set to 1.
However, in combination with other cvars “r_uifullscreen 1” can be exploited, this is why it is recommended to be restricted to 0.
It is not recommended to ban a player for this setting, but if you do, you probably will ban an innocent player.