Denmark set a precedent when they stripped a man of his citizenship after he was convicted of terrorism. This is a precedent that should be followed by other countries: when an individual commits a terrorist act, they are violating the rules and regulations of the country to which they belong, and therefore should no longer be considered a member of that country.
Terrorism is an unthinkable crime, one of the worst that can be committed. I believe that no country wants to provide citizenship to a person who has committed such a horrendous crime. The person will most likely be put into prison, so it wouldn't even matter if they were a citizen of any country.
This actually sounds like a fantastic plan. Then he has the protection of no country and he can be a stateless phantom with no rights or certainties in life. This is actually a very ingenious punishment. We should do this with more people than just terrorists, too. I am sure it would work.
No matter where you live, no matter the rules, the final decisions and judgments you make in life are YOURS. If YOU choose to become a terrorist or complete terrorist activities, YOU yourself have made the decision to go against rules in your country. This would be one of the strongest ways to convict someone is to never allow them back in the country. But then what do they do with that person? It won't be their problem!