Just talk to them and tell them it's not the weed itself that annoys you, it's the constant sound and the smell. Ask them nicely with something like "mate, I know you just want to have a good time and I've got no problem with that but can you just keep the noise down and try and do it in your room or somewhere where it won't leave too much of a smell?".
If they persist, tell them that if they do it once more you'll just report them. If they do it again, then just report them.
The people telling you to report them are wrong. Fine say you did, what would happen? They would get a first and final warning perhaps or just a talking down to. You've still got to be living with them for at least the rest of the academic years - if they're smart they'll just tell everyone in the house that you grassed them up, making you both unpopular immediately and creating a lot of awkward moments. It's really not worth it.
My advice would be, that reporting them should really be a last resort - they may not realise how annoying it is to everyone else when they're stoned off their heads and if you explained why it annoyed you, it's likely that they will stop. However, if it does come to reporting them, make sure you explain it to your house mates first, like telling them you've asked them to stop a couple of times and it hasn't worked. That way when the smokers try and turn everyone against you, it won't work.