Since you have 12.9 you can start to use incident transitions to show buttons e.g. when incident is resolved. This should also make reopen button to disappear on closed incidents
This is statement from employees detai_in.htmpl that makes me think that button should disappear.
// Do not display this button if using status transitions.
if (!bUseTransitionButtons) {
ImgBtnCreate("btn003","Reopen_Incident", "reopen_incident()", "defer", 0);
}