To clarify, in the JobRoller Admin Dashboard you have the option to "Enable Job Seeker Registration", so you can actually disable this part without customization of the theme. Job Seekers do not need to be registered in order to apply for jobs.
If you basically don't want anyone else to be able to create job listings, but you still want people to be able to register, then yes you would want to make some customization to the theme in order to remove the "Submit a job" button. However, you need to consider that you won't be able to collect payment for job listings made using this method, at least not via JobRoller.
AppThemes does not officially support customizations, however as a customer you can seek community based support in our "Mods & Tutorials" sections in the support forums.