Solution is simple. Lock everything down.
Put a new password on the computer so no one can use it, and turn it off when she comes.
Get a lockbox and put everything in there (games, DSi, everything you don't want her to touch), preferably a box that uses a number dial.
When she comes to visit, take out a book, sit on the couch, and read. Do nothing else, just read.
She'll get bored out of her mind eventually and she'll stop coming.