OK Adamastor.
In our company we do it slightly different from what Flygirl describes. People whom the powers-that-be have decided are not wanted in country are accepted on the national carrier under certain conditions. They are graded as to the level of danger they supposedly pose. The first category, ranging from non violent illegal immigrants to prostitutes having outstayed their "tourist visa", travel unaccompanied. Chief FA gets handed their papers by the authorities who deliver these pax to the plane. As soon as we're in the air, we hand back the documents to the owners. This because we are NOT the authorities nor an extension of them. Never had any problems yet with this category. They are usually totally peacefull, get treated like any other pax by ourselves and are often allready planning their return trip!
Second category travels accompanied by 1 or more police officers. Police officers handle the papers and the pax. Danger here has been shown to lie in the other passengers. When the original criminal gets violent, often his countrymen decide to come to the rescue. And then there isn't a hell of a lot 2 police people and some FA's can do. Pretty scary in fact.
I've also been on flights were people were transported handcuffed all the way, again accompanied by law enforcement. Who then are supposedly responsible for O2 masks and evacuation in case of emergency.
If you want to know more, feel free to mail me.