Have you tried this link? I'm pretty sure this is the answer you're looking for, this link tells you to;
1) Open the Mac Keychain. Click Finder > Go > Utilities > Keychain
2) In Keychain Access, select the Passwords category on the left to
review the list of sites and services for which you've saved a
3) Right-click the service you want to edit/remove, then left-click
If you're looking to get rid of sandbox,
1) Quit sandbox as well as its related process(es) if they are still
2) Launch Finder on your Mac, and click Applications in the Finder
3) Search for sandbox in the /Applications folder, then drag its icon
to the Trash icon located at the end of the Dock, and drop it there.
Also, you can right-click/control click sandbox icon and then choose
to Move to Trash option from the sub-menu.
4) If prompted, enter the administrator password and hit OK to allow
5) Choose Finder > Empty Trash from the Menu bar. Or choose
right-click Trash icon, choose Empty Trash, and Empty Trash again in
the pop-up dialogue.
(Note, you don't have to install any apps to remove it)
If you want to reset the entire keychain, I recommend that you take a look at the answer on this page.
How do I delete or fix my Keychain after changing my password?