You can call Avis First & have them put a note on your profile: "Do not upgrade." Also, if you call them the morning of your rental & ask them for exactly what you reserved, without upgrading you, you should have better luck. Also, at smaller locations, when pulling up your reservation, they see the 2nd line in your address profile. You can put something in there too.

However, standard Avis policy is that you can be 'upgraded' to anything in a higher class than you reserved. Your options are 1. Take what they give you, or 2. Walk. It's very unfortunate because I have reserved premium cars & get a Ford Edge/Escape as an upgrade. Also, just because you put a note on your account doesn't mean anything. I have a "No SUVs or Trucks" in my profile, and just today got myself a Ford Expedition... it was that or nothing. Good luck!