Not certain if ideas such as proposed here will be helpful in the situation you cite:
• Time Machine - Frequently Asked Question #12 - Should I delete old backups? If so How?
http://pondini.org/TM/12.html
The above is from this site, which still has much helpful information:
• Apple OS X and Time Machine Tips:
http://pondini.org/TM/Home.html
Usually Finder-based solutions in this matter are not recommended or just don't work.
So at least by attempting to go through Time Machine itself is a correct path to try.
Sorry I don't have enough experience to venture further in this matter. Perhaps another
person with experience deleting specific user account backups from Time Machine will
offer their methods or means toward resolving this issue.
And if you don't already, create duplicate backups, to not rely on Time Machine alone.
To different media and not the same storage drive; in case of failure and so on.
In the search results, I've found by looking, some direct to wordpress sites and those
are not necessarily worth the trip; some redirect to other places. I reported them to
the duckduckgo search people. And some of the older Apple Discussions results may
or may not be helpful. Here's one that covers user account backup in Time machine:
Remove specific files from Time Machine backups
Good luck in this matter! 🙂