Unfortunately it doesn't look like there's a way to do that from the computer. The block feature is tied specifically to your iPhone itself, not your iPhone's SIM card or your iCloud account. This is something for Apple to consider changing, considering iCloud has enabled multiple access points of communication... Time to go make a feature suggestion on the Apple.com/Feedback page! 😀
**EDIT: Here's an idea. You can choose which medium Messages receives iMessages, so if she's only got the phone number associated with your iCloud account you can simply disable it in Messages' preferences.
In Messages, go to Preferences > Accounts > your iMessage account. Unclick the phone number and you should be free & clear, unless she also has your email and sends you iMessages there, in which case you're stuck until Apple creates a fix. This also means your laptop won't get any iMessages your other contacts send to your iCloud-associated phone number, but you can let those friends know to specifically point their messages at your iCloud email.
