While the other answers are correct, you should note that they will delete any matches. For example, active_user = "John"
will remove "John", "John123", "OtherJohn", etc.
You can use regular expressions to test, or if user names don't have parentheses, do your test like this:
string comp = active_user + "("; // The ( is the start of the date part
activeUsers.RemoveAll(u => u.StartsWith(comp));
Also note, this is case sensitive.