How to punish beginners for playing the Sicilian?

Yeah, that's what I do.

I was just asking if there are any tricks or specific things to look out for.  I've just heard about how beginners shouldn't play the sicilian, meaning that they should be getting into dodgy positions that I should be able to exploit.  

I don't think that's why it's not recommended. It's not that the positions are bad, it's that it doesn't follow opening principals, and can lead to complex games. It's still chess of course, but it's not a good way to learn fundamentals because being behind in development doesn't get you killed, or certain pawn structures aren't necessarily a weakness... so you can get the wrong impression about positions in general if the sicilian is all you've played as a newer player.

Fundamentally as well, it could be said that c5 on its own is very passive, and when followed up by d6 can allow me to occupy a lot of the centre.

You can't get the pawn duo with d4 and e4 because c5 guards d4 of course. But yes, it's very common in some variations of the Sicilian that white gets more central space and better development. The Sicilian is played at the highest level though, so it's not fundamentally bad. Black generally has no real weakenss, is flexible to choose how to set his center pawns, and has pressure on the c file and queenside.

I understand that if they actually played one of the main lines (such as the najdorf for example), the sharpness would come with the different variations of white's attack, however a lot of people leave theory within 5 moves.  I was wondering if anyone could give me tips about what sort of things I should be looking to do in these situations?

It depends. If they leave theory really early, or play very odd looking moves, then you just focus on fast development, castling, and maintaining a pawn in the center. That's what new players are told of course (the opening principals) but it's really the way the opening should be played. If their development is less efficient, if they have less space, if their king is less safe... then the punishment comes in the middlegame, not immediately in the opening. You can exploit most of these advantages by playing a pawn break to open lines in the area of the board (queenside, center, kingside) where you have more space and better development.

If they leave theory a little later on, usually not much will have changed in terms of the general ideas. So if you saw a GM game where white attacked on the kingside by pushing the f and g pawns, then that's likely still the main idea. Just be sure to not fall behind in development, castle to safety, and then go for it in the middlegame.

Yeah, that's what I do.

I was just asking if there are any tricks or specific things to look out for.  I've just heard about how beginners shouldn't play the sicilian, meaning that they should be getting into dodgy positions that I should be able to exploit.  Fundamentally as well, it could be said that c5 on its own is very passive, and when followed up by d6 can allow me to occupy a lot of the centre.

I understand that if they actually played one of the main lines (such as the najdorf for example), the sharpness would come with the different variations of white's attack, however a lot of people leave theory within 5 moves.  I was wondering if anyone could give me tips about what sort of things I should be looking to do in these situations?

I was just hoping someone could tell me how to punish beginners for playing such a complex opening they know nothing about.