You can leave any Workspace you are a member of from it's Overview page, here is a guide that will walk you through what to do. Remember that if you have left a Workspace and you want to see its contents again, you will need to be invited back in by a member.
1) Select the Workspace you want to leave from the list
2) Select the Three vertical dots to the right of the name of the workspace
3) Select Leave workspace
4) You will no longer be a member of the workspace and it will disappear from your workspace picker menu