If these teacher comments are the ones attached to Final Grades, you can go to Start Page > School Setup > Parent/Student Access and click the Available Features tab where you can choose to disable final grades. This will be for ALL Final Grades, however.
If this is not the case, or this does not work, than unfortunately PowerSchool does not have the ability to remove the Teacher Comments from the public portal. If you would like we can put in an enhancement request for you so this can possibly be implemented in a future version of PowerSchool. I'm sorry we do not have a better answer currently.