It is normally in the Block Services section not the Products and Apps area of your MyVerizon account online.

As for completely removing the Visual Voicemail feature from your line, you will need to call customer service at 1-800-922-0204 from a landline or *611 from your phone and request they add the Block Visual Voicemail feature to your line(s).

As for the PMs, you might have to reply through the forum instead of replying to the email notification of the private message (PM). If the rep does respond, they can also add the Block Visual Voicemail feature to your line(s). One of the VZW reps that roam this community helped me to add it to my account over a year ago.