> Skip all SLA notifications on stalled tickets
INSERT INTO KBSYS.SETTINGS (NAME, VALUE) VALUES ('HD_SLA_EMAIL','suppress_stalled_all');

> Skip recurring notifications alone for stalled tickets (Notification will be sent just once)
INSERT INTO KBSYS.SETTINGS (NAME, VALUE) VALUES ('HD_SLA_EMAIL','suppress_stalled_recurring');

Removing this row or updating with empty string will restore the default behavior.