Under your My Workspace tab click on Membership. You can then click the Unjoin button under the appropriate site you wish to disassociate yourself from. You can also email someone that runs the site to request to be removed.

If you want to stay in the site but hide its tab, login to bSpace and click Preferences in the toolbar. Click the Customize Tabs link at the top of the screen. Sort your site tabs with the arrows and then click the Update Preferences button. You will still receive site email with the tab hidden.