Response.Cache.SetCacheability(System.Web.HttpCacheability.NoCache);
Response.AddHeader("pragma", "no-cache");
Response.AddHeader("Cache-Control", "no-cache");
Response.CacheControl = "no-cache";
Response.Expires = -1;
Response.ExpiresAbsolute = DateTime.Now.AddSeconds(-1);
Response.Cache.SetNoStore();
Page.ClientScript.RegisterStartupScript(this.GetType(), "cle", "windows.history.clear", true);
Or Use Javascript
<script>
function noBack(){window.history.forward();}
noBack();
window.onload=noBack;
window.onpageshow=function(evt){if(evt.persisted)noBack();}
window.onunload=function(){void(0);}
</script></script>
Check this link for more information