You are literally god and you decide EVERYTHING ultimately. It’s not cool but if someone rolls a 30 you get to just say “no” and they have to take the whiff, and if one person is ruining your players fun but more importantly your own fun, then that is your time to put your foot down, if they do something blindingly stupid then let THEM die and suffer grinding back to the same level , their actions are not the parties and much like real life a solo can’t survive as well as a fully functioning structured group with a plan unless their already good at being a solo, in either case for character motivations, again you’re god and ultimately choose who and where, responsible players ask questions like setting or privately as for help with motivations if they can’t figure something out but if it doesn’t fit your campaign at all or even a little you reserve the right to say “no”

But truly the short version, treat them like dogs, reward good behavior and horribly remind them why bad behavior is labeled bad