If getting the renters out before the closing was a term of the contract, then the seller is in breach and you should be able to walk away from the deal as well as recover (by lawsuit, if necessary) your costs. If however getting the renters out is not actually a term or clause of the contract of sale, then seller has not breached--even if they made verbal representations that they would get rid of the renters, if that's not in the contract of sale, that contract still stands.
This is a situation where you should consult with a local RE attorney for a definitive answer, even if it costs you something for the consulation--the stakes are very high in home-purchase cases like this.