It's possible with the built-in Search & Replace feature using regex (just don't try it on too large a selection at once, or be prepared to kill a locked up MusicBee from taskmgr). This search criteria string should remove the "Rock" genre tag, while still keeping any other existing tags intact (keep 'Replace' field blank):
(?<!(\s|\w|-|\.|\\|/))Rock(?!(\s|\w|-|\.|\\|/))
Of course, test it on a small batch first and always review the preview of changes, but worked like a charm for me!