This is how it works.
First you need to have an "Personal Shop".
Put the item/s that you want to remove in the personal shop.
Note: Doesn't work if the items is unclickable (eg. some summoner scroll)
Then after putting it, you only need to "Level Reset" your character and then...
the items that you put in your personal shop will be gone.
Alternatively solution...
1. Create a character.
2. Put the item/s the you wish to remove in that character (use the storage for trading).
3. Delete the newly created character along with the item that needs to be remove.