A phone sweepstakes fraud is harassing my dad with calls. How can we get them to stop?

I have found that the minute a senior replies to something in the mail or, as your father did, send money, those companies flag them and become ruthless (there should be a hot rock waiting for them in h*ll.)
Another scam to be aware of: My mother odered something from a catalog. They sent her the item and a few months later a small "refund" check which she cashed. Then she got another one so I read the fine print. It said, "by cashing this check you are now enrolled in a entertainment program." Nothing happened for a long time, then, six months later, her credit card was charged $160.00!! I called the company that made the charge and they referred me to the catalog co. I called and gave them a piece of my mind for targeting seniors - then I called the credit card co. and had them suspend the charge.
Another related thing you can do (especially if your parent cannot take care of their own finances - or you have a pilfering relative) is to place a "fraud alert" on their credit report. I think it lasts for 90 days. In that time no one can access the report or take out any loans etc. in your parent's name. The lending institution has to see picture ID before processing the loan, etc.
Third: Good luck getting rid of these leaches. To save your sanity and time, you may have to have your father's number changed to unlisted. Also, is it possible to have all his mail go to you or to a PO box so you can screen?
I have learned over the years that there are millions of scam artists out there who are just waiting to pick the pockets of seniors.

You can go to the postmaster general at the post office and also put a block on those letters coming through the mail. The same thing happened with my folks. You may also be able to find the form you need to fill out online at USPS website. It's been a while since I've done it but that will give you a start.

Thanks Kelly!
They're on the Do Not Call list. And I submitted a complaint to the FTC with all of the phone numbers/call times I found on their caller ID. Not sure what I can do to keep them from calling right now. I'm going to try to get in touch with the phone company today.

There's is a national do not call list that you can be added to. If they call you within a certain amount of time, the company or whoever is penalized. If you tell them not to call you i forget what happens then, but afterwards its considered harassment.