Per Blackbaud support, the only way to rename a user (for example, someone that got married or divorced) is for that user to login and change their name themselves. An administrator cannot change a user's name and, in fact, cannot stop a user from changing their name.

We also cannot change a user's e-mail address. Which is a problem since, using the Google SSO, the user cannot login to change her name or e-mail address. We actually have to turn off the SSO – for all users – then help the user login using Blackbaud credentials, then reenable the SSO.