How to deactivate devices authorized for my Adobe Digital Editions ID?

When I try to activate my new device I receive an error message E_ACT_TOO_MANY_ACTIVATIONS.

I assume I've reached the limit of 6 devices. I only have one current other device, probably most of the other activations are on old devices which are broken or trashed. So unauthorizing those old devices from the Adobe Digital Editions menu is not possible anymore.

Is there any other way to unauthorize the old devices?  Adobe Supoort chat doesn't know and point me to these froums.