One of the ways you could do this is to modify the dialogs / logic so the user is not prompted to choose between change and remove. Also, modify the registry for Add/Remove Programs entry, adding the NoModify / NoRepair entries.
I now created a new project for my product and selected in the project wizard the option "I prefer to provide an uninstall option only".
I got the code for maintenance:
label: Maintenance
Comment: Maintenance Install/Uninstall
Set Variable UNINSTALL to YES
MessageBox: $TITLE$ Uninstall, Are you sure you want to uninstall the selected application and all of its components?
if Variable UNINSTALL Equals NO
Terminate Installation
else
Set Variable REMOVE to TRUE
end
I pasted this code in my standard project and now it works as desired.
Would be easier if you could choose this option also if the project wizard is already finished.