How to clear list of servers "Choose a mac" list in OS X Server App
On the client mac (non-server):

Quit Server.app

Activate Finder, hit Cmd-Shift-G and navigate to ~/Library/Preferences

Find "com.apple.Server.v4.plist" and change its name to "com.apple.Server.v4.OLD.plist" or something similar. 
Move it to the Desktop if you wish, but don't delete it so that you can easily reinstate it if necessary.

When you reopen Server.app it should present the first-time splash screen, asking what you want to do.
Select the option relating to 'another machine'.