1. Eating Meat at Langar: Serving meat at langar is strictly prohibited. The Rehat Maryada does not specifically ban the consumption of meat nor does it condone it. It does however state that "Eating the meat of an animal slaughtered the Muslim way;" is strictly prohibited. This applies not only to Halal, but any ritually slaughtered meat, including Kosher. The Hukamnama issued by the Akal Takht stated that meat killed in the Jhatka fashion (one blow, or as painlessly as possible) is the only meat that should be consumed by a Sikh. There are however some Sikh groups who advocate a strict vegetarian life style . Unlike Hinduism, Sikhism does not specifically prohibit the consumption of beef.
2. Cutting Hair: Cutting hair is strictly forbidden in Sikhism. From your head down to your toes, no hair is to be plucked, cut, burnt or chemically/surgically removed. Kesh (hair) should not to be dyed by any method whatsoever.
3. Intoxication: Alcohol, drugs, tobacco, and consumption of other intoxicants, etc. is not allowed.
4. Adultery: In Sikhism, the husband and wife must be physically faithful to one another. People who allow lust to overcome them, and violate this rule, will not escape the circle of life and death.
5. Some spiritual behavior: Superstitions and rituals not meaningful to Sikhs should not be observed or followed;
   
   
   
   (pilgrimages, fasting and bathing in rivers; circumcision; worship of graves, idols, pictures; compulsory wearing of the veil for women; etc.). Compare to the Five Ks of Sikhism.
6. Material obsession: (“Maya”) Accumulation of materials have no meaning in Sikhism. Wealth, gold, portfolios, stocks, commodities, and properties will all be left here on Earth when you depart. Do not get attached to them.
7. Sacrifice of creatures: Sati – widows throwing themselves in the funeral pyre of their husbands, lamb and calf slaughter to celebrate holy occasions, etc. are forbidden.
8. Non-family-oriented living: A Sikh is encouraged not to live as a recluse, beggar, yogi, monk, nun, or celibate.
9. Worthless talk: Bragging, gossip, lying, etc. are not permitted.
10. Priestly class: Sikhs do not have to depend on a priest for any of the functions that need to be performed.

References