You could try blocking the field from updates in Field Management:
but you would risk losing valid information - such as when someone's office phone number changes, or if you currently have a bogus number and they actually want to update it with a valid one.
Is Phone required on the form? This may just be an understandable reaction to too many required fields on a form, which marketers should be wary of. Too many required fields and a form seems less like an opportunity and more like an irritation - people will just fill it in with junk information to get to whatever gated content is behind the form. It's always great to ask for more information, but requiring it can encourage leads to enter bad data.
It is possible to block field updates in Marketo from specific sources. So you could block updates from form, but not from SFDC, which would allow your salespeople to update them even while forms could not. This can be done in Marketo Admin > Field Management. Search the name of the field you want to block on the right hand side, and then drag it to the middle and choose Field Actions at the top.
Please note that even blocked fields will always accept the first value provided, but thereafter will block updates from the sources you've chosen.