I notice that neither the KJV or NLT say this. 2 Thes. 3:15 in the KJV is "Yet count him not as an enemy, but admonish him as a brother." In the NLT, "Don't think of them as enemies, but speak to them as you would to a Christian who needs to be warned." I checked the NRSV too, and it says words to the same effect.
Verse 14 seems to be what you're thinking of, which states (NRSV): "Take note of those who do not obey what we say in this letter; have nothing to do with them, so that they may be ashamed." Even then, I went to the Greek text, and it doesn't support your notion that we are to "stand firm in the traditions..." The KJV, NRSV and NLT all line up with the Greek text. So what translation are you using? New American Bible?