Kind of thinking outside the box - what if you added a custom node property, such as "AlertOn?" and then in the edit node properties, set it to 1. Then, in your rule, part of the trigger condition is that the the custom node property "AlertOn?" equal 1.
When you want it NOT to alert, you go into Manage Nodes/Edit Multiple Values select your "AlertOn" and clear them all.
It's still managed, it's still monitored & reported, but the alert trigger condition isn't met any more, so no alert.