I'm not exactly sure which bar you're talking about, but I'm going to take a guess. If you mean the shortcut bar which you can carry three items on, either bought before a match whilst waiting in a chamber or picked up during the match, you can easily remove the items in-game or whislt in a chamber.
To remove items whilst ingame all you need to do is hover your mouse cursor over it and you will see a little red 'x' appear, click on it to remove a particular item.
To remove items whilst in a chamber all you need to do is double click on the item to remove it.
Hope that helps bud