Go to Exchange Admin Center
Select the SigParser Mailboxes Mail Enabled Security Group
Select Members
Select View and manage members
Check the box next to each mailbox you want to remove from SigParser, then select Remove members
In SigParser, go to Mailboxes and, under the "Connection" column, select the Office 365 Admin Connection
Select the Resync button at the top of the page
The removed mailboxes should no longer appear in the list at the bottom of the page. SigParser stops processing them from the next scheduled sync.