I am totally new to Moodle but have a similar situation. After searching the net, I have found your advice, but I am not sure I fully understand.
As you suggested, I have created my own Plug-in Folder with a db/access.php file and a lib.php file. Is the access.php file supposed to be like a Configuration file or what is it supposed to do? You then write the following:
"Firstly do a 'has_capability('name of the new capability you created', $context)' to check if the user is able to add see the buttons at all. Next, as a site admin, you can override the role definitions at the level of the category, so that only 'course creators' (or is it 'editing teachers') within that specific category (and sub categories and courses) have that capability.
Once you've established the capability, you can then look through the contents of the '$params' variable to remove the 'moodlemedia' button, if it is found in any of the 'theme_advanced_buttonsXX' parts (unless they do have the required capability)."
Could you explain this a little bit more? I really appreciate any help, as I said, I have never come across Moodle before and it is still rather confusing. Thank you very much in advance.