Adobe reader

Adobe Live Chat: http://www.adobe.com/support/chat/ivrchat.html

They can reset your authorizations, and then you must reauthorize any devices you still need.

(Unfortunately, Adobe haven’t got round to an admin website for viewing and editing authorizations.)

Some of the representatives haven't been properly trained and don't know what to do (and claim there is nothing they can do);

in that case the only way seems to be to give up that chat and try another session hoping for a properly trained representative.

If your problem is with another device using Overdrive, Bluefire, Aldiko or similar third party app, it is recommended not to mention that app when on the chat, just mention that you have run out of authorizations  (E_ACT_TOO_MANY_ACTIVATIONS) .  Thanks to AJP_Bear for that tip.