If you need to remove the Build Manager plugin you can do so by using the Application Management feature in the Notes client as well by removing an entry from the Notes.ini.
1) Remove the Notes.ini setting ‘$TMSBuildManagerPluginVersion’.
2) Launch the Application Management UI. i.e. File/Application/Application Management
3) Select the Build Manager plugin and choose to uninstall.
4) Choose 'Ok' and restart your Notes client.
*Note: If you don't see the Application Management menu item you may need to enable it by adding the setting 'com.ibm.notes.branding/enable.update.ui=true' to your plugin_customization.ini file located in <Notes Program dir>/framework/rcp folder. However, if this does not work you may need to speak to your Domino Administrators to see if its disablement has been performed via Domino Policies. You can find out more at the following IBM support article here.