Is it possible to kill someone through thought alone or by following certain rituals?

You must remember that Derren Brown is an illusionist. This doesn't mean that certain effects that he replicated (remote viewing for example,) on tv doesn't exist for real. It only a trick that he's giving people (power of auto-suggestion). Easy to get sucked in isn't it if you haven't researched the subjects and don't know what has been found in these certain fields.

So, what do you think - can curses work?

Yes, without any doubt. The answer is 'belief'. Belief can have strange effects.

If a Witch doctor is making one believe that voodoo can kill s/he then that belief will kill you.

If your belief in the effect is strong then the effect then it will work.

How about placebo? can sugar tablets heal? Doctors fool people everyday belief systems that these tablets (placebo) will help you. The thing about placebo is that Doctors have completely no idea what going on. Can one healing one's self though the power of auto-suggestion?