Is it possible to prevent an audience or an individual user from accessing a category or all courses in a category?

We are on a multi-tenanted site that uses categories for each organisation and have a case where we want to prevent an individual user from being able to sign up to any seminars in our courses. 

We thought perhaps we need an audience of blocked/banned users, and then we might be able to find a way to prevent that audience from accessing our seminars. 

We can do it with restrict access on toipcs or activities but there is no restrict access at category or course level that we can identify. 

e.g. Not available unless you are not a member of the audience blocked users (hidden otherwise). Not particularly keen to do this on an individual seminar or topic basis.

Thanks for any ideas from the community.