How to disable the validation using validation controls

Hi
I have  registration form and  username is  checked in database
if there is user name in database,the message "user name already exists" should come first.
and then all the validation controls should be disabled.
but i get all the validation controls first executed and then message "user name already exists" comes.
Please help me to fix the error.I have given code below.
Thanks
Chandran
<asp:TextBox ID="txtUserName" runat="server" TabIndex="1" onfocus="Javascript:return test()"></asp:TextBox> <asp:RequiredFieldValidator ID="rfvUserName" runat="server" ControlToValidate="txt ...