Siebel : How to disable an applet button?

Just edit your applet server script and add some come for WebApplet_PreCanInvokeMethod.

For example:

function WebApplet_PreCanInvokeMethod (MethodName, &CanInvoke
{
switch (MethodName)
{
case 'Metodo1':
if (true)
{
CanInvoke = "TRUE";
}
else
{
CanInvoke = "FALSE";
}
}
}

In this case Metodo1 will be the method being invoked by the button that you want to disable.