One of the best ways I've seen webmasters avoid chargebacks is records.

What IP did that person login as (each time)
What days? What did they download?

Then you write a nice letter...

Dear member:
I see you requested a chargeback for your membership.
I also see that you have accessed the site 29 out of the 30 days.
Downloaded (x) amount of files.

We follow up on all chargebacks and will subit our information to the credit card company and police as it appears that your card was stolen. If in fact you did use our site please make arrangements to void the chargeback or pay us what owe in the next two days.

Of course, this is not what they send out as I'm not at the office. BUT the thought is on track.

Keep records and then follow through.
If a person kept signing in on one or two IPs each day and downloaded or viewed files...then go after them.