Hi Phil,
A standard remove link message code can be used for these users just as it can for those added from a signup form. The default !remove code should exist in "Message Codes" -> "Global Codes". You may want to click "Edit" next to the code to modify it's options, text, etc.
As always I recommend giving it a try on a test list to make sure it works as expected before sending to a live list.
Regards