Members of the Church of Latter Day Saints are forbidden from having heated drinks.
In scenes reminiscent of Netflix hit Orange Is The New Black, crafty cons said they were Mormons to get the sweeter beverage to drink in their cells.
Figures show 1,121 lags said they were Mormons in 2013.
But once the realised it was a scam, that figure fell to 737 last year. There are 751 self-identified Mormon inmates but only 15 are practising.
One prison chaplain said: “We invited a Mormon preacher in for worship and suddenly they didn’t seem as interested.”