The following sample code shows how you can check if the current user action is banned for the current site. It uses a Label control with the ID lblError to display the error message:
[C#]
| using CMS.SiteProvider; using CMS.CMSHelper; using CMS.GlobalHelper; 
 ... 
 // Check if current action is allowed if (!BannedIPInfoProvider.IsAllowed(CMSContext.CurrentSiteName, BanControlEnum.AllNonComplete)) { // If not error message is shown lblError.CssClass = "ErrorLabel"; lblError.Text = "Requested action was canceled. Banned IP address."; } | 
Page url: http://devnet.kentico.com/docs/5_5r2/devguide/index.html?api_checking_if_actions_are_banned.htm