The word "be-foul" means to make something dirty, so the antonyms would be words that are associated with cleaning or purification. Some possible antonyms could be "cleanse," "purify," "sterilize," "sanitize," "decontaminate," "disinfect," "wash," "rinse," "scrub," "polish," "purge," "clarify," "refine," or "purge." These words all suggest the opposite of making something dirty, and instead imply that something is being made clean or free from impurities. Using one of these antonyms in place of "be-foul" would convey a different meaning entirely, one that suggests a process of cleaning rather than dirtying.