No. They are not able to kill with guns either. They can not kill at all. It is part of the genetic makeup. If they know their physical action has death of another human as end-result they get the seizures and are unable to continue.
The only way is trough excessive treatment like the one John Young has had, or by ordering (or in other way telling a normal human) to do it.
They can also kill by accident.
SPOILER (Ep. 20 Season one)
In episode 20 Hillary Cole straps a bomb to herself and asks Astrid Finch to call a number to set it off. Astrid does not know this is why she should call. Hillary places herself beside Hugh Bathory (The Founder) as the bomb goes off. This, IMO, breaks some of the above statement as it is a conscious choice to kill someone. She should not have been able to go into that room etc.
It is revealed that the purpose for The Machine is to stop time for normal humans, and as such they are not technically alive when frozen. As a result The Tomorrow People can kill the humans while they are in this state.