As of version 1.5 of USP Pro, it’s possible to exclude categories on a per-form basis.

To do so, just add the exclude attribute to the [usp_category] shortcode, like so:

[usp_category exclude="1,2,3"]

Then specify whichever categories you would like to exclude from the global USP settings (located at General ▸ Categories ▸ Post Categories). The excluded categories should be a list of category ids separated by commas if more than one. Any excluded categories will not be displayed on the USP Form.