One reason why the Jehovah's Witnesses are wrong in their belief that the hell mentioned in the Bible is not eternal:

"The Son of man indeed goeth, as it is written of him: but woe to that man by whom the Son of man is betrayed! good were it for that man if he had never been born." [Mark 14:21 ]

If all the unsaved have to look forward to is annihilation (not eternity in hell) then Jesus would never had said the above statement. Annihilation is not "punishment". A loving God would certainly punish people in hell forever if that God loves righteousness and hates sin as we read here in the Psalms:

"The foolish shall not stand in thy sight: thou hatest all workers of iniquity. Thou shalt destroy them that speak leasing: the LORD will abhor the bloody and deceitful man." [Psalms 5:5-6]