Drill through the styles of the adjoining cabinets,countersink the screw heads,clamp the spacer(if possible,remove the doors which might be in the way,clamping is important!)
Slowly screw the piece in,remember, most spacers are hardwood(and if painted, its probably not the most desirable wood to begin with...knots,etc)
If you over tighten the screws, they'll break off and you get to do it again...
Hope this helps,
Ken