From time to time you may need to perform maintenance work on particular VMs. During this time you would prefer for these VMs not to be used by Jenkins.

Click on Pooled Virtual Machines, click on a (static or folder) pool, and click on a machine in that pool. Just press the Take Offline button to prevent Jenkins from reserving it. (You can enter a description of what you are doing for the benefit of other users.) Later when the machine is ready, go back to the same page and press Take Online.

Offline status is remembered across Jenkins restarts.