Find the preference file
(workspace/.metadata/.plugins/org.eclipse.core.runtime/.settings/org.springsource.ide.eclipse.dashboard.ui.prefs)
And manually add (or change) the line:
org.springsource.ide.eclipse.dashboard.uidashboard.startup=false
Make sure to:
- backup the prefs file in case you accidentally damage it
- edit the file while STS is closed.