Software can be maddening when it does what you tell it to do rather than what you want it to do.
go to the properties and then on the visibility tab, select "Show on Printed Page " = Never.
I wouldn't. The performer needs to see this Key change. Something like this:
!NoteWorthyComposerClip(2.0,Single)
|Clef|Type:Treble
|Key|Signature:Bb,Eb,Ab
|TimeSig|Signature:Common
|Chord|Dur:Whole|Pos:-2,0,2
|Flow|Style:Segno|Pos:10|Placement:AtNextNote
|Bar
|Chord|Dur:Whole|Pos:-4,-2,1
|Flow|Style:ToCoda|Pos:10|Justify:Right
|Bar|Style:Double
|Key|Signature:C|Visibility:Never
|Key|Signature:F#
|Chord|Dur:Whole|Pos:-2,0,2
|Bar|Style:Double
|Key|Signature:C|Visibility:Never
|Key|Signature:Bb,Eb,Ab|Color:1
|Flow|Style:DSalCoda|Pos:10|Justify:Right|Placement:AtNextNote
|Bar|Style:Double|SysBreak:Y|Visibility:Never
|Flow|Style:Coda|Pos:10|Justify:Right|Placement:AsStaffSignature
|Key|Signature:Bb,Eb,Ab
|Chord|Dur:Whole|Pos:-2,0,2
!NoteWorthyComposerClip-End
(color added for emphasis)
If there is a change of Clef or time signature, the perfomer will need to see that as well.
NoteWorthy doesn't do a very good job of setting Codas off from the main body of music. Sometimes post-processing needs to be done as in the attachment.
edit 2009Apr20: Changed KeySig order and some placement options in clip.