Under products and services ensure you are looking at the correct number then click "Manage My Add-ons". This is found, in blue, on the extreme right hand side of the page about halfway down.
* Update 2015*
You will be able to Remove the value pack however to exclude the voicemail only from the value pack you would need to call into Rogers customer service at: 1-888-764-3771 and they will be able to exclude the voicemail only from the value pack. If the voicemail is an individual add on without the value pack then you will have the option to remove it through the "Manage My Add ons".