there is no way to "unregister" nor recover MAK activations. Once activated it is consumed.
Note that a MAK activation is not equivalent to a license. Your license "pool" (assigned licenses plus unassigned licenses) is not related to "activations consumed".
An MAK has a certain number of activations assigned to it. If you require additional activations, or additional MAK's, you can request these via MS VL team.
You can check total/available actiovations for an MAK by checking at your VLSC website logon or by using the VAMT.