The quickest and easiest way to bulk unsubscribe contacts is to import these contacts from a CSV file into a new group.
The following article highlights the steps you need to take to import a new group from a CSV file.
Then, follow these steps;
1. Click the contacts tab
2. Click the name of the group you just imported
3. Depending on how many contacts you have in that group, either choose the 'view all' option at the bottom of your screen or the '100 / page'' option if you have more then 100 contacts within your group
4. Click 'select all'
5. Click 'unsubscribe contacts'
6. Confirm this action
You will only be able to unsubscribe 100 contacts per page. If you have > 100 contacts, you will need to follow the above steps until all contacts in the groups have been selected.