You can delete one or several collections from My Collections section.
In order to delete a collection, it has to be hidden first. To hide a collection, make sure that it is not set as your default collection. Check out more about default collections here.
If these conditions are met, you will see a Delete collection option. Once you select this, you will have to confirm that you want to delete the collection by typing DELETE + collection name (e.g., DELETE My Pokémon collection)
Keep in mind that you can’t delete a collection if it’s the only collection you have. Make sure you have at least two published collections by the time you delete one. Having at least one active collection is required.