Go to the plan to watch tab Select the shows you want to delete /or/ click Ctrl + A to select all /or/ Shift + Left Click on one title and while still holding Shift, Left Click a title below it to select a bunch Right Click + Delete from List
Full Answer
To delete anything and everything on your MyAnimeList account, just skip the form, go straight to deletion mode, and click start deleting. Take note that content deletion is irreversible. This is recommended if you want to have a fresh MyAnimeList account rather than making a new one.
Just as everyone else said, press on edit details and a thing that says delete will appear, just click on it and it will be removed from your list.
Unfortunately you have to delete them one at a time. Quickest way is to be on your list on desktop. Click "Edit" on an entry you want to delete, then click "delete," then "ok." Click the X on that popup then go to the next entry and repeat.
You can't delete your account. Use the tip below to obfuscate your personal data. There used to be a way to remove all your lists in one go, but due to perfomance reasons this option is removed.