Short answer: Don’t! Just kidding :-)

Long answer:

I believe it is the same process as for other Microsoft Online Services – but I have never tried it. Hence please post a comment if you follow this successfully or not and I will amend.

From http://www.microsoft.com/online/help/en-us/mocp/, search for “cancel” and you get:

What I am not clear about is whether an Introductory Special is classed as a trial. Either way, the answer is to contact support and ask to cancel. I would suggest you are fully armed with details of your subscription which you can get from signing in to https://mocp.microsoftonline.com.

You can contact support via a online web form at https://mocp-support.custhelp.com/

Or

You can call them.

The details are again on the support page http://www.microsoft.com/online/help/en-us/mocp/ 

In the UK you can call 0800 731 8457 or (0) 20 3027 6039 Monday – Friday 09:00 – 17:00 GMT (UTC).

I hope that helps.