How can I remove a browser's cache server-side, using ASP.NET (C#)?

A coupon shows by itself (I believe it comes from cache as I did also browse for other apparel sites). It breaks my JavaScript as well as my server-side code, as I am using an UpdatePanel for Ajax, and it duplicates the UpdatePanel's ID. I have renamed the UpdatePanel's ID, but it makes no difference. It generates "Invalid view State" exception. The coupon name is "FastSave"

What I have tried:

Response.Cache.SetExpires(DateTime.UtcNow.AddMinutes(-1));
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.Cache.SetNoStore();