How to remove 'add to resource to project' system prompt

When you create an activity associated with a project, and it's assigned to someone that IS NOT added to the 'Project Resources' detail (which is a list of contacts that can be assigned to the project) the system will pop up a prompt that asks 'Do you want to add user as a project resource?'

We don't use that object on Projects, so this is confusing our users. How/where can we eliminate it?