Mark,
That depends on where your users are now. Chances are the users are at Seq 1 Delay 0 so they will receive new followups added to the list.
There should be a "Reset Users" option on the Followups page. Clicking that provides the option to set the Seq and Delay for all users on the list. Set the sequence to one more than your last followup and all reset users will only receive new followups if you add them. Set seq very high (999) if you never want them to receive followups.
Please let me know if I can be of further assistance!
Regards