You can cancel online BUT it takes effect immediately so if your going to use it for the whole month then I would strongly suggest that you also cancel the direct debit at your bank so that they cannot claim any money.
It's worth noting that in order to successfully cancel a Direct Debit before it's due you should do it by noon the working day before it is due to be taken.
On the off chance you leave it too late the DVLA should refund you, some people have had a refund quickly and others have had a bit of a wait. If you really don't want to wait then you can ask your bank to reclaim the money under the direct debit guarantee. They usually credit your account the same day and then reclaim the money from the DVLA