Place first 7 ingredients on a double thickness of cheesecloth. Gather corners of cloth to enclose seasonings; tie securely with string.
In a large stockpot, bring water, salt, lemon juice, cayenne and spice bag to a boil. Using tongs, add crab to stockpot; return to a boil. Reduce heat; simmer, covered, until shells turn bright red, about 15 minutes.
Using tongs, remove crab from pot. Run under cold water or plunge into ice water. Serve with melted butter and lemon wedges.