If you want to hide the Sign in Link which is displayed into the welcome control, what you have to do is to put the Welcome control into some div and assign ID to that div. After that use style=”display:none” in that div. Now the main thing is to put below code after the closing of the welcome control div
<Sharepoint:SPSecurityTrimmedControl ID=”SPSecurityTrimmedControl” runat=”server” PermissionsString=”AddAndCustomizePages”>
document.getElementById(“id of div containing welcome control”).style.display = “block”;
What the above code is doing is displaying the welcome control div to only users having”AddAndCustomizePages” permissions and not users having least permission on site.