The most direct way is an option you turn on while importing. You can also clean up prospects who've gone quiet after they're already in a campaign.
Filter as you import (easiest)
When you add prospects, turn on Remove prospects who haven't posted in the last 30 days. Right after adding, Extrovert checks each person's recent posts and removes the ones who have not posted. The same option is in the import form, in Lead Finder and in Suggested prospects.
In Lead Finder the check runs after the people are retrieved, so credits are charged for everyone found, including the people it removes. It gives you a smaller, active list, but it does not save credits.
Import from a Sales Navigator search
If you build your list in Sales Navigator, add a recent-post activity filter to your search before importing, so only active posters come through:
Open your Sales Navigator search
Set your target criteria (industry, role, location)
Add the recent-post activity filter
Paste the search URL into Extrovert to import
For all the import methods, see the complete guide to importing prospects.
Clean up prospects who stopped posting
Already imported people who don't post? On your prospect list, open the ... menu and choose Delete inactive prospects. This removes everyone who hasn't posted in the last 30 days, and it can't be undone. Deleting them frees room in your team's prospect limit. See deleting inactive prospects.
Prefer to keep them? Instead of deleting non-posters, you can still reach them by engaging with posts they're likely to see.