Currently the only way for a marketing contact to be removed from the quota limit is to have no interaction with him for 1 year (which is quite long for marketing purpose).

Even if you delete a contact, it will not be removed of the marketing contact quota and worst if he comes back within 1 year in the DB and you have an interaction this contact it will be count in fact twice (the one deleted and the new one created).

In the current set-up there is no way to manage your quota except buying new sku. Microsft should offer us a way to manage quota like e.g removing deleted or inactive contacts from the quota limit.