@jpmitchell ...
I'm not exactly sure if the comment library is supposed to work that way or not. Something tells me that what you described is accurate...as I somehow remember a different conversation here in the Community a long while back, but I don't remember exactly where it was that I saw it. ☹️
Is there a way to disable it?
Yes...there is a way to disable this...but it is at the Canvas account level. This means that if your Canvas administrator turns it off, it will be turned off for every course in that account. While you might not want to use it, other of your fellow instructors might really like using it and would suddenly wonder what happened to it ... if it was turned off.
The Feature Option at the account level is called "Comment Library", and the description is, "Allows users to save and search frequently used feedback in a library of comments to be used in SpeedGrader." The options are either "Enabled" or "Disabled".
I hope this will help a bit. Sing out if you have any other questions...thanks!