kafka connect consumer group members with no assigments available

Question

I have a kafka connect task which fetches data from a topic with 3 partitions and send the data to a cassandra sink, so I have kconnect in distributed mode with max task up to 3. This is fine, it works really great. But last week I saw more members in the consumer group than the max tasks. It doesn't look an error because I see 3 assigned and the rest are shown as "No assignments available". So it seems like old used clients.

Why does this happens?

How can I self-purge these old members?

Can I delete these, not anymore, used members?

Here you can see a screenshot of the members of the topic, the dark blue ones in the column assignments are the "good" working ones, three and reading from different partitions 0,1,2.

The rest are the "unused" with no assignments available.