It can be really annoying but I think if you want to put anything in your listing you will need to word it carefully and you will still get some buyers wanting you to lower the price or end early.

"I do not close auctions early so please do not waste my time or yours by asking."

Yes that is a little hostile, but just a bit of rewording will make it alright. Never mention wasting time as that will always come across as narky.

Something along the lines of 

Please understand I am unable to close auctions early.

Or something similar, there are plenty of posters that will be able to word it much better than me.