Great post. I do work for free sometimes. When I feel like it. When I want to. Not because I feel like I should. Many times I say no, ignore, delete, politely respond it's not a good fit, ask if there is another campaign that has a budget we could work on, etc etc. 

I wrote this post which has also helped with communications as well. 
http://www.greeblemonkey.com/2011/11/psa-for-pr-professionals-and-their.html

No need to get rude, upset, or mean. But ABSOLUTELY okay to say no.