When to give up seeking justice after you've been plagiarized?
As you know, there are two separate issues with plagiarism: 


Claiming ideas as one's own
Claiming words as one's own


You have a legitimate concern about the second -- if this student is going to directly use your words, he needs to use quotes. Attempts to get around this by making one or two insignificant changes are not appropriate. 

That said, I can understand why the president of the university didn't want to get involved -- as academic crimes go, this one is quite minor. I agree that the "lack of intent to deceive" ruling was erroneous, which is unfortunate; getting the student to re-write the content, with no other penalty, would have been a far more appropriate outcome. 


  At what point should I give up seeking justice? 


I think it would be appropriate to reach out to the student (I would have started here, personally), their advisor, and the university's ombudsperson (in that order), just asking them to rewrite those sections and to not do it again. If they refuse, it would probably take a lawyer to get them to listen -- but you don't have any damages (not even to your reputation), so that seems a bit excessive in this case.