Since there are multiple steps needed from both sides, to accept and come to an agreement, during each of which each side has the option to cancel, a cancellation after the fact is currently not being directly offered.
However, it is possible to request a cancellation which will trigger a message to the other side in the agreement. If both sides agree, then the license is cancelled, but if the other side does not - for whatever reason - agree, the license and more specifically the deal and any costs associated with it remain valid and must be paid.
A "request cancellation" button will show up next to the agreement, once a deal has been struck.