I could use some DU advice on "early cancellation" fees

Last year I called an exterminator to deal with a persistent ant problem. Ants were chewing on wood next to the foundation.

They refused to do just one visit. I had to sign up for a years' worth of treatments. Four visits. They warned that if I canceled before the four visits were completed and paid for, they would impose a whopping "early cancellation' fee on me. Something like over $300.

The fourth and final treatment was in October, so I assumed we were done with them.

A couple of weeks ago (just when I was reeling from the sudden death of my brother and the need to replace the main sewer drain in front of the house) the pest company sent me an email that their next treatment visit would be January 17. I told them I no longer wanted their service.

They sent me an email demanding a $150 "early cancellation" fee. They tried billing my credit card, but the bank sent me a new card recently, so they couldn't get the money. Next they sent a letter by mail demanding payment.

Unfortunately, I can't find any written contract with them, and am at a loss as to what to do.

Why should I have to pay an early cancellation fee -- for what? They didn't provide any service, beyond the 4 treatments I agreed to last year. This seems very unfair.