My husband's aunt has dementia and she sneaks on the computer answering emails about loans. How can I stop her?

If it is your internet account you can block all incoming emails coming into her email address except for the ones you chose to allow in the email settings.

PASSWORD PROTECT THE ENTIRE COMPUTER.

When she has trouble getting on, just tell her, Hmmm, must be a computer glitch and that you'll check into it when you get a moment.

When she's sleeping, go on her email account and delete everything that isn't a personal email. Then let her get on and handle her correspondence.

If that doesn't work, you'll just have to password protect the computer and pretend that it's permanently broken.