I would not do that.
I rather clients drop me and I've never had to fire a client. But as most that know me will tell you, I have no problem being direct and firm.
I charge $85/hr. If I wanted to charge for after hours support, it would probably be $150/hr.
If she was my client, I would email, fax or SNAIL mail her my new support terms.
The first time I received a call from her, I would ask if she received and agreed with my new terms. If she doesn't agree then that would be that.
If she has not received the agreement, the support part of the call would not start until I either emailed or faxed the agreement on the spot and waited for her to read it, or I would read it over the phone and wait for her to agree to it.

Credit card would be taken at the beginning of a phone call if I didn't trust the client to pay. Otherwise, a bill could be sent during next business day.

Any unpaid/outstanding invoices (and related late fees) have to be paid before current support issues is addressed.