Sprout Forms - Possible to reduce available field types in form builder
Sprout Forms for Craft 3 only displays fields that can be used on the form. 



Craft 2 answer:

In Sprout Forms for Craft 2, it is not possible to toggle the visibility of the fields available, it displays all fields that can be received by the form.

Sprout Forms does break out the fields listed into option groups, so with a training-based approach, you could show a client that only the Standard Fields are supported and that all the Custom Fields require a developer to get implemented properly.

Another approach could be to use a plugin like Control Panel CSS and hide the Custom Fields option group:

.sprout-field-modal .fieldtoggle [label="Custom Fields"] {
  display: none;
}