Identify the perpetrators but don't sack them immediately. Investigate them thoroughly. Assuming they're well paid, resorting to carting off a fridge full of soda every week is a disturbing way to behave and I'd want to understand their motivation and identify if they behaved this way in other areas of their work-life. They likely thought (1) they'd not get caught or(/and) (2) the value of the theft was so low that they'd likely not get in trouble. I'd be concerned most about the "I won't get caught" people at any company that a large part of their profits are from selling closed-source software. Sharing loads of source code might also be seen as something they'd think they wouldn't get caught doing. I'd still be concerned about anyone with the moral flexibility to brazenly steal large quantities of even inexpensive items and expect, if caught, to get away with it. At a minimum, that's not the kind of culture you want to spreading and it probably speaks to their intelligence slightly.
Generally, people hate "passive-aggressive notes". At a previous someone placed a note on the sink asking everyone to clean up the microwave after using it. Someone had heated up pasta and left the tomato sauce all over the interior. I took the note down and kindly asked the person responsible to be more careful in the future. It was equally effective, but without pointing the finger at every person on the floor. In the case of the soda, skip any note and let the word spread through informal lateral communication ("gossip" channels are rarely used effectively by management).
Funny, no one has ever put up a sign to tell me not to tear down the signs.
I've learned that passive aggressive can work both ways. ;-)
Identifying them for further investigation will generally turn up more ethical and performance issues, safely justifying reassignment or outplacement.
2. Sack perpetrators.
3. Put a notice on the drinks machine that, through use of polite informality, makes it clear what was going on and that it would be appreciated if people didn't take the mickey. ("Inform, don't instruct" should be a rule of management.)