By phoning VM on 150 (VM line, fixed or VM mobile and free) or 0345 454 1111. You won't be cancelling your contract. You will enter a new 12 month contract for broadband only. You can never be out of contract if you want VM services. You must always have a contract between you and VM.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.