you can use CSS to hide buttons. Unfortunately most of the formatting buttons don't have unique class names, but you can target the title or aria labels instead. You will need to adapt the code if your forum is in a different language. For example (not tested, but I think it should work):
.TextEditor-toolbar button[aria-label="Mention a user or post"] {
display: none;
}
This won't prevent inserting mentions manually however.
