How to omit content types

I'm afraid that option didn't get added in yet - you can only omit vocabularies or users.

I also needed this functionality.

They way i achieved it is by using access control, roles and groups as follows:

1) Create roles
2) Create Groups
3) Uncheck the "audience" checkbox in the og configuration
4) Create content and allow it to be seen by the relevant groups
5) Create users and assign them the relevant roles.
6) Control access to the content types via access control.

The theory is as follows:
Groups limits what users can view, access control limits what users can create.

Let me know if this is not clear and i will try help.

lp