I am having the same issue. We have a customer who is on a paid active smart list and they canceled their subscription and yet they are still qualifying for this smart list, so I am trying to remove them however, just like in your case, the only option is 'remove from list' which doesn't help since this is a smart list. The option of using the filter of email address and using 'not in' doesn't work because that is a band aid and doesn't resolve the issue, especially if other smart campaigns are used with the same list. Remove from Flow also doesn't work because that is campaign specific and once again is a band aid, it needs to be removed from the actual smart list. I did some research and apparently Marketo does not have a way to remove a person from a smart list, which to me makes no sense but only to exclude. Everyone seems to be using the same solution 'not in email address ....'. as the only way to exclude people from a smart list.
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