Firstly, they've changed the settings for creatures now, so that Elite/Arch Villains can spawn amongst mobs (they didn't in the past), so what you need to do is go to the My Enemy Groups tab, and RIGHT CLICK on the creature so it says "Don't Auto Spawn"
Secondly, if you're getting 5 of them, it sounds like you've got the Ambush set to hard 
It won't matter once Don't Auto Spawn is on, but if it's Hard the chances of something above Boss level turning up seems to be quite high too
You currently can't set an ambush to have a Unique character in it and guarantee that there will only be one of him. An ambush needs to be of an enemy group.
Eva Destruction AR/Fire/Munitions Blaster
Darkfire Avenger DM/SD/Body Scrapper
Arc ID#161629 Freaks, Geeks, and Men in Black
Arc ID#431270 Until the End of the World
The problem is Eva that if the Unique character is part of that Enemy Group, he can turn up anywhere it appears if there's a gap in the roster... I had an Elite Boss turning up amongst normal mobs as a Lieutenant in my arc, and they'll appear in ambushes too if not set to Don't Auto Spawn. Once that's on, and the Ambush is set to Single, it should just be him... but if he has any associated spawn, he may replicate into that as long as he's Auto'd.
The problem is also that you cant just set a boss fight to be spawned as an ambush.
That would solve a lot of problems, and me a lot of headaches and workarounds.
Does anyone know why I would be unable to set "Don't Auto Spawn"? For the life of me, I can't figure it out.
Topher Wade lvl 50 Claws/Regen
The Crimson Heroes Society SG
Chaos Faction VG
Official Naturalized Citizen of Justice since 2007
Nevermind, I figured it out. Was clicking on character in left box, needed to click them in right box.
Topher Wade lvl 50 Claws/Regen
The Crimson Heroes Society SG
Chaos Faction VG
Official Naturalized Citizen of Justice since 2007
Why, when I have set up an ambush with a unique character in, is there 4 or 5 of them that turn up? This is driving me nuts.
Rgds
@Boy Wonder
Quantum Flash - Ill/Kin Controller
Leader of the Legion of Heroes