Yes you can cancel Anytime.
So if you want only one month useage, cancel in the first month (a couple of days before the next due payment date, where you still get the full month).
Also it comes with a free one month Boost Membership, so again make sure you cancel Boost as well.
My example above was if you only wanted the Sports offer for one month.
Should you want it for a second month for example, then cancel the Sports in month 2 (again cancel a couple of days before the next due payment) and you will only pay for two months useage and will not be charged for any further months.
There is roughly five cancel steps to fully cancel and once cancelled it should say next to your Membership and usually you will also receive an email confirmation from NOW that you have cancelled a Membership.
With regards to Boost Membership you need to decide if to cancel in month 1 and get it for free for the first month or continue with Boost in month two onwards and pay a further £6 per month for Boost until you decide to cancel Boost.
When you cancel Boost you may get a retention discount offer for Boost to continue with the Boost Membership.
Yes if you only want the free one month Boost then cancel it straight away in month one and you will not be charged for the Boost.
Also if you wish, when you go to your checkout screen when you are going to purchase the Sports offer, there should be an option to remove Boost if you don't want the free one month Boost.
Personally if it was me I would take up the Boost one month free and cancel it straight away to take advantage of the features which Boost entails.