Temporarily un-associate term for an amount of time

I think since the users are a CPT, it would be well suited to do this in post_meta on the user. You could store the temporarily unsubscribed term there and check against when sending notifications.

For example, your post_meta could be an array such as this:

subscribed_list_term => "maybe_cool_for_summer_list_term", subscription_duration => "halted_during_all_seasons_but_summer"

Obviously that is abstracted, but there is logic that can be built upon this idea.