Cancel DSL but still charge for it. Claim it takes 3 months for the money you paid for a service you didn't get to be refunded.
Give a free dial-up account that costs $19.99/month.
Don't hire enough operators to answer the phone so that it takes 20 minutes waiting on hold to get through. Encourage people to email questions but then tell them their question can only be answered over the phone.
Get people to sign up for automatic payments so you can get the money you think they owe before they get to see the bill.