I had read "15 punishments for leaving prayers", then later read on http://www.themodernreligion.com/misc/hoax/punish.html that it was a fabricated Hadith and that there was no truth to this Hadith. There was no reference number for this, so I was unable to check its authenticity. Is this a fabricated Hadith?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

As you mentioned, the Hadeeth that states 15 forms of punishment for leaving prayers is fabricated as reported by more than one of the scholars of Hadeeth.

Imam ath-Thahabi in his book Mizaan al-E‘tidal, while writing down the biography of Muhammad ibn ‘Ali ibn al-‘Abbas al-Baghdadi al-Attar, said: ‘He (Muhammad ibn ‘Ali) fabricated on the authority of Ali ibn Bakr ibn Ziyad al-Naysaboori an ascription of a false Hadeeth concerning one who neglects performing prayers’.

Ibn Hajar said in his book Lisaan al-Mizaan: ‘This Hadeeth is clearly false, and it is fabricated among the Sufi groups’.

Therefore, the Hadeeth you have asked about is a fabricated Hadeeth and it is not authentic.

Allah knows best.