The most effective method for registering your opinion is by phone, then follow-up in writing by email on his contact form or by fax. Request a response.
If you use Twitter, you can also direct message @JeffFlake but social media is the least effective method so do it last or not at all.
Contact Form on Senate website (in lieu of email)
Washington, D.C. Office
Senate Russell Office Building 413
Washington, D.C. 20510
2200 East Camelback Road
Phoenix, AZ 85016
6840 North Oracle Road
Tucson, AZ 85704