Negaitve keywords are an effective way to keep your pay-per-click ad from showing when someone searches on unrelated keywords.  For example, if you sell women’s shoes, you don’t want your ad showing when someone searches on mens’ shoes.  So it may seem logical to add the negative keyword, men or men’s, to keep your ad from showing on any search term that contains men.  However, this probably won’t keep your ad from showing for “men’s shoes”.

According to Google, a single negative keyword might not achive the results you want:

Your ads still might show on variations of the terms that you select as negative keywords. Your ads might still show on searches that contain only one of your negative keyword terms if your keyword terms are more than one word. For example, if you add the negative keyword “wine,” your ad could still show to people searching for “stemless wine glasses.”

So, the best solution is to add negative keywords that are relevant to phrases you want to block.  Adding the negative keywords, men’s shoes, men’s sandals, men’s sneakers will probably keep your add from showing for any search related to men’s shoes.