Just remember, that your neighbour can open a criminal case of theft and/or damage to property against you. If you are caught poisoning a pet, let's add a number of charges violating the animal protection act as well as a few fines from the SPCA. If you happen to use two-step, a few more charges as 2-step is a controlled and illegal substance.
My suggestion is to inform the neighbour and then use a water gun / garden hose. If you manage to catch the cat in your house, drop it into the shower, turn the water onto cold and let it run for a few minutes - the cat will never, ever come back again. With cats it is always a territorial thing and it makes no difference if the cat is spayed/neutered. Just don't be a d**k and kill someone's pet (similar to some people on the forum, the cat might be a jerk away from home but still belongs to some family). If their owners don't look after the pets, phone the SPCA.