Here is a from the Discord guidelines page
Do not promote, coordinate, or engage in harassment. Discord is a platform where everyone can find a place to belong, and harassment prevents users from building healthy communities. We do not allow harassing behavior such as sustained bullying, ban or block evasion, doxxing, or coordinating server joins for the purposes of harassing server members (such as server raiding).
Because what you are describing is likely quite rare it doesn't sound like anything is specifically against it unless they are joining your server specifically to try to get people out.
If they are doing it on their own server or elsewhere it would likely be hard to enforce.