ok.. I think I read all of this, and even followed it. so.. my take:
showing them the ad and saying they owe 650 isn't going to work, because the ad says 630. still a better position to be in than 600.. accepting the originally advertised 630 is the least acceptable solution.
can't cancel the dsl, because it's not in your name. you CAN change your mind about paying for it though.. you're not using it, why should you pay for it? probably though, in coming months, that's just going to amount to.. 15-17 bucks or so.. not a huge amount, you can live up to that agreement with no real pain, providing they live up to theirs.
cutting off the utilities is rash, but would certainly get their attention. the real downside to that is they're undoubtedly going to have reconnect fees.. more money that's got to come from somewhere. the other possible problem with that is that it may not be legal to shut off the utilities in an apartment (may be a violation of the lease). throwing the appropriate breakers might be a reasonable and legal tactic.. no electricity in their bedrooms. really really really hope you don't have to go there, there is NO way y'all are going to manage to live in a civil way for the rest of the year if you do that.
sit down and tell them that there appears to be a misunderstanding as to what was agreed to. explain to them that your understanding, the conditions underwhich you invited them to share the apartment with you, were that they would pay 650 a month, which included rent and their share of the utilities (hence the "utilities included" statement). show them paperwork that backs up the fact that utilities are always at least 150 and frequently over that, proving to them that the 50/month share is no more than 1/3 of what the actual bills are. providing that's true. if they're ever less than that then demonstrate that at least if averaged over 12 months, utilities amount to more than 150/month. explain to them that you cannot afford to, nor is it fair to expect you to pay 100% of the utility bills each month, that your room is not $150/month larger than theirs.
As a compromise, you can offer to pay 630/month for your room and reduce their rent to 585/month each, to reflect the difference in room size, and then treat the utilities in the same manner you're currently handling the DSL.. everybody pays exactly 1/3 of the actual bill each month. that's going to be the best/fairest/most equitable solution.. you're still better off than you were under the advertised 630/month deal, they can see they're paying less rent for smaller rooms, they can see exactly where their utility money is going. they're paying (most months) less than the 650 total, you're better off than you would have been if they were paying 630 as originally advertised and way better off than you are right this minute.
ok.. I think that's perfect. each party is giving something. so nobody feels especially taken advantage of, and nobody feels especially like they won something over the others, either.. evens the playing field, makes way for y'all to be good roommates.
once you get that settled, ask them to please stop eating your food.