Limit the amount of categories to members to post

Let me explain my problem:
– I have a site on which all members can make new posts
– There’s 6 categories of post types, members are allowed to just use 2 of those categories
– Posts by members are made through the front end by using this plugin: ‘post from site‘

Here’s the problem: I can not make it work to show just the 2 categories members are allowed to post in.

Any help is welcome, I’m not limited to this plugin though posting from the front end is mandatory.