The only reliable way to do it, considering that the user's name or ID is not an attribute of the HTML element that the comment is in, or any of its parents, is to use either an experimental implementation of the :has() relational selector (which no browser natively supports), or experimental XPath.
Both of those things are
under consideration for ABP, and both
already exist in uBlock Origin.
A sketch of how :has() would work is something like
youtube.com##div.comment-renderer:has(>div.comment-renderer-content>div.comment-renderer-header>a[data-ytid="CHANNEL_ID"]), where CHANNEL_ID can be found by right-clicking the user's comment and picking Inspect Element.
For XPath, you could craft a similar query for finding "a div with class
comment-renderer that has an immediate child that is a div with class
comment-renderer-content...", or (if you wish to allow false positives), you can figure out how to target the element based on its text content, which cannot be done in CSS; as with :has(), you need a specific filter (at least one domain left of the ## part), and the format after the ## part is
:xpath(XPATH_QUERY).
A couple of decent sources for info. on crafting XPath queries are
W3Schools (not affiliated with the W3C but much more understandable than
the spec) and
MSDN.