I am not aware of a global way to do this in the 3CX PBX interface, however if the issue is that you have a lot of extensions in this system and wish to save time simply go to the "Extensions" tab in 3CX and hold down "Ctrl" and select multiple extensions and "edit".
The system supports the ability for "Block use of 3CX Client" as you have said. This can be found under Provisioning >> Access.
This option is available when in the multiple extensions selection state and should at least offer you a quick work around.
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