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To delete a particular transition, select the transition and simply press the delete button on your keyboard.
To delete them all, just press Command + F on your keyboard. Then enter the name of the transition (eg Dissolve) and press Delete or Command + Delete
Remember, this is going to delete all selected transitions and even the ones applied by you.
To prevent this happening, we can reset the preferences of FCP to default ones.
In your Mac, on finder, Hold down Option key on your Keyboard, click on Go menu and select Library. This will take you to the Library folder of your User Account. Open Preferences folder.
Trash the file com.apple.FinalCutPro.plist from there
. Then trash the Final Cut Pro User Data
folder. Don't empty the trash. This will reset some settings of your FCP. Please let me know if that didn't work, so we can continue troubleshooting.