Document, repeat, witnesses, document, witnesses, repeat.
I was given very high marks, for terminating a gay subordinate around 1976-77.
His performance dropped off very badly, and he started wearing makeup.
I accepted a promotion with the proviso that I immediately deal with him. I told my superiors I intended to warn, document, and make it about performance, not his lifestyle choices.
When I lowered the final hammer, the guy thanked me.