I know this is old but...
Since this was the first thing that came up in Google search results for "How to delete multiple monitors PRTG", I'll provide some input after figuring it out.
Go to "Devices", select "Management" from the secondary menu, CTRL/Shift click the devices like you would like using a windows browser.
When you have the selection correct, on the right hand side of the webpage, you'll see a list of all of the monitors you have selected. Below those you'll see a trashcan. Select that and confirm.
Devs... When I was using chrome, the confirmation box could not be moved, and was not displaying the Confirm/Cancel buttons at the bottom. I also could not scroll down to see them. I ended up having to zoom out quite a bit and then reset the zoom back to %100 after clicking the confirm button.
- Allow users to right click when multiple devices are selected in managment and select delete (currently only deletes the one device you happen to right click on with the others highlighted)
- Allow us to scroll down or move the box or something so that we're not having to adjust the zoom to see the buttons.