Simply call 0345 454 11 11 (150 if you have a Virgin Landline or Mobile) and speak to someone in Retentions by following the options 1,1,4,4 - they'll be able to remove it and possibly provide a discounted service (if you haven't already got one). If you do take a new offer then you might possibly be put into an 18 month contract term so best ensure it's definitely within range of your finances to prevent putting yourself in a worse situation, if you decide to just remove the box you'll either save £7.50 or £2.50 depending if the box is discounted by a £5 credit.
Recommended call time is at 8am as to avoid any queues and get it sorted ASAP.

** I work for VirginMedia but all opinions posted here are my own.