First of you is your issue related with RadWindow as you have posted the problem under this thread? In order to improve our customer support, we advise our clients to use the common thread for problems not related to a specific problem. Basically, you can create a validation and check whether the user has already submitted the form, and if not to inform him by using a MessageBox or RadWindow modal dialog. If this doesn't solve your issue, feel free to contact us for further assistance.
the Telerik team