Yosemite - Finder has a disconnect button when attached to OSX Server, not working
In the Finder, select the "Go" menu > "Connect to Server...", then click on the Recent Servers menu button (button with tiny clock, to the right of the "+" button) and select "Clear Recent Servers".

Now go to a Finder window, select the server from the list and click on the Disconnect button and the server will be disconnected.

(OS 10.10.3)