If Chaser is unpaused, all customers with outstanding invoices will be chased by default. If you don't want a particular customer to be chased, you can do this via the customers overview page and select "Stop Chasing".
In the customer overview you can also set all customers to not chasing.
This will move that customer over to the "Not Chasing" side where you will be able to start chasing them again by selecting "Start chasing" in that column, or "Set all to 'Chasing'"
Selecting "Stop chasing" or "Start chasing" for a customer will also change the chasing status for all their invoices, to manage invoices individually you can go into the customer page and under invoices select start or stop chasing on the relevant lines.