BBC NEWS | Africa | Nigeria's gas profits 'up in smoke'

For Shell to say they cannot do anything is somewhat disingenuous, they (Shell) had exactly the same problem in Oman.
Where prior to the restrictions on the burning off of Gas from Oil wells, flares were between 3 to 10 metres tall, after restrictions came into force, these flare stacks reduced to around 1/2 Meter maximum.
If Shell were able to effect this reduction in the Oman in the 1980s I see no reason why they cannot do the same in the Nigerian Delta Region.