Using disabled option, pushbutton can be disabled.
pushbutton( [1,30], "Create Material","/nmm01", {'disabled':true});
//Dynamically disabling the pushbutton:
z_test = true;
pushbutton( [1,30], "Create Material","/nmm01", {'disabled':z_test});
Note: This option is available from Liquid UI server version 565 and Liquid UI Desktop version 326