Of Swine and Fools

It is easy for one to believe he is able to convince even the most obstinate individuals. Unfortunately, this is often a sign of pride and self-righteousness, rather than charisma or rhetorical skill. Scripture offers the following advice in regard to correcting and/or reproving others.

"Do not give dogs what is holy; and do not throw your pearls before swine, lest they trample them under foot and turn to attack you." – Matthew 7:2

"He who corrects a scoffer gets himself abuse, and he who reproves a wicked man incurs injury. Do not reprove a scoffer, or he will hate you; reprove a wise man, and he will love you. Give instruction to a wise man, and he will be still wiser; teach a righteous man and he will increase in learning." – Proverbs 9:7-9

Apparently, some people are not worth spending too much time and energy on, but does that mean we should just give up on them? If so, when and how? Certainly, we should be humble and prayerful in our argumentation and be vigilant for signs of fruitlessness in dialog. I wonder, though, how we are to know when a debate is not worth continuing and how we are end it without feeling guilty for abandoning someone, particularly a brother/sister in Christ, to his/her grievous errors. Thoughts?