In which case write to them.
Don’t deal on the telephone. Tell them that they are on their own admission in breach of contract as they admit that the account has not been run well.
Tell them that you are now cancelling with immediate effect and not paying them. Tell them that if they feel differently they should issue legal proceedings which you will defend.
Then a judge will decide if you are correct or they are.
The most they should get as they haven’t provided the service for the full period is loss of profit in any event, not the full value of the job
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