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Delete the App from Applications, and delete the folder at /Users/You/Library/Hydrus. That library folder is all my code writes to permanently. If you want to be very thorough, clear out your OS temp folder, as there may be some old temp files hanging around if you have had crashes.
I am not an expert at macOS. Perhaps there are also some system shortcuts or history or something that is added at the OS level. If you know macOS does this and you want to completely wipe all memory of hydrus, you'll have to search how to do this.