Deleting a monitor from this page is reflected instantly and the devices that will be associated with that template in the future, but it still remains in all the devices that have been already associated with that template. To apply the changes to all these devices, click on 'Save and Associate' button in the Edit device template page.
Only custom monitors created by the users can be deleted from this page.
Deleting a custom monitor from here removes it permanently from OpManager, and from any device/device template that has this monitor configured already.
Removing it from the device snapshot page will only de-associate that monitor from the particular device and will not affect other devices or the device template in any way. You can also bulk delete multiple monitors by selecting them and clicking the bin icon (Delete selected row) below the monitors list.