Only way I could think of is to set a really short period, like 00:00 to 00:01.
Then you only need to change the latter back to 23:59 and its enabled.
Don't believe there is an easy toggle in the app.
EDIT: another thing I just thought of: you could also turn the notifications off for DMSS in the phone itself. On my Android I simply press-hold the DMSS icon, click 'App-info', Notifications, don't allow.
This way the notifications will still be visible in the app under 'Message' but you won't be bothered by them.
Might even be able to make it a short cut setting by some third party app.