How To Disable RequiredFieldValidator In Script

I have TextBox with RequiredFieldValidator on my page.I also have link that calls some simple javascript.

<asp:TextBox ID="TextBox1" runat="server" />
<asp:RequiredFieldValidator
ID="RequiredFieldValidator4" runat="server" ControlToValidate="TextBox1"
Display="Dynamic" />
<asp:LinkButton ID="Link1" runat="server" OnClientClick="DoSomething(); return false;" CausesValidation="false"Text="Do" />
function DoSomething() {
textbox1.val("blah"); }

When user type something into textbox and then delete that text and focus next control - then validator fires.Then user can use link that adds text using javascript. TextBox1 text is no longer empty but RequiredFieldValidator still shows error message. How to prevent that?