Contacts Hygiene means cleaning your contacts/sequences/lists so that you keep only those contacts who want to hear from you.
After you've identified which contacts are unresponsive, you have a few choices:
1. Delete them
2. Add tag
3. Add tag and send broadcast email to these contacts
#1 is self-explanatory. You just delete them. That's it.
For #2 and #3, you could tag them as "unresponsive" and send a broadcast email letting them know if they still want to hear from you, they'll need to click the link inside the email (e.g. http://yourwebsite.com/want-to-hear)
Then you set up a rule like this:
which basically means if contacts click the email link, you remove the "unresponsive" tag from them. This also means those still with the "unresponsive" tag do not want to hear from you.
The next time you send a broadcast email, you can exclude these "unresponsive" contacts by using the "exclude tags" feature.
To make sure they also don't receive sequence emails, you set up another rule like this: