Tortious interference of contract?

We own our own company. An advertising company solicited us because our competitor trashed us and he wanted us to advertise not our competitor and he said we would be the only company like ours in his ad. A few days later, we heard our competitor was making police complaints about advertising, so we called the company to see if he was calling on them because we wanted nothing to do with it if he was calling on them. They said he was just there and smelled like a bar and made the women in the office uncomfortable. They said he was begging to be put on an ad. They said they did not have room for him because they already have a company like his advertising. A month later, we found out the ad company was falsely promising where the ads would be and got a full refund. A few days later they wrote a letter for our competitor saying they only cancelled him because we slandered our competitor. Now our competitor might be suing us for tortious interference of breach of contract. Is this even possible? How can you defend against hearsay? We have copies of the check we got as a refund, but we never slandered him, they did! Help!