By setting the disabled property of tab to be true, the user could not select or close the corresponding tab by clicking the tab or close button. But, Developers can still control the selection or close of tab by program.

<tabbox width="300px" id="tbx">
    <tabs>    
        <tab label="Step 1" id="tb1" disabled="true"/>        
        <tab label="Step 2" id="tb2" disabled="true"/>        
        <tab label="Step 3" id="tb3" disabled="true"/>        
    </tabs>    
    <tabpanels>    
        <tabpanel><button label="to Step2" onClick="tbx.selectedTab=tb2"/></tabpanel>        
        <tabpanel><button label="to Step3" onClick="tbx.selectedTab=tb3"/></tabpanel>        
        <tabpanel>This is panel 3</tabpanel>        
    </tabpanels>    
</tabbox>