There was a checkbox 'Omit background graphics from master' in the 'Format
Background' option for the slides in version 2003. Not sure where it has been
moved/removed in 2007.
yes, you can remove a master from 1 slide only.
Right click the slide ( Not on slide, this is on left. On the slide number).
Click layout and choose > Office Theme (since 'office' is the default theme
as soon as you open powerpoint and that’s default.)