Keith Inc.

“Thanks, but your editorial efforts are not necessary.  I have three editors who will beet me with a riding crop every time i make a grammer misteak.  So far, they have not begun to suspect that i disperately NEED those beetings!  STOP CRAMPING MY SEX LIFE!”

Frankly, i don’t think you sound like an asshole if you say, “Thanks, but I already have an editor.”  
Link them to the forum where they can offer their services as an editor.
Write a chapter where an uninvited editor is eviscerated by rabid deer ticks.  LOTS of rabid deer ticks.  Maybe as a side note, or something reported on the TV in the background, or a story told around a campfire.  Misspell Camphire.  Make the uninvited editor’s last act on earth to be correcting the spelling of camphire.  If they still don’t get the connection, then all bets are off and it’s time for chainsaws.
 
 

“Thanks, but your editorial efforts are not necessary.  I have three editors who will beet me with a riding crop every time i make a grammer misteak.  So far, they have not begun to suspect that i disperately NEED those beetings!  STOP CRAMPING MY SEX LIFE!”