I know when an aircraft has a clearance and you give them traffic information as a heads up as to why they have been levelled off below their requested final level, and also to help them with when they can expect to get further climb, I never say "traffic crossing from your right to left 5 NM FL280". I would always say "traffic crossing from your right to left 5NM 1,000 ft above". The reason is two fold for this, one is to avoid the transmission being possibly clipped or over transmitted and the pilot hearing their callsign and a FL, and going ahead and climbing to that level. Also in many parts of the world the level of English comprehension is basic to say the least so these pilots are known to hear and understand only the basics and again, may jump to the wrong conclusion and start to climb to said FL.
I know the giving of low level traffic information is a little different, but I was always taught the blanket rule, that you should never say a FL or Altitude unless you are happy for the pilot to climb to that level/altitude. Same goes with when a pilot says "request FL360", my response would never be "FL360 not available", I would always say "requested level not available", and tell them what is available, or just sayer "higher levels not available" and give a reason if time permits.