An "SP" is a "Suppressive Person", a phrase invented by L. Ron Hubbard, creator of Scientology. Critics of Scientology were declard enemies by LRH. He declared that SPs were fair game:

"ENEMY - SP Order. Fair game. May be deprived of property or injured by any means by any Scientologist without discipline of the Scientologist. May be tricked, sued, lied to or destroyed." - L. Ron Hubbard, Oct. 18th 1967 (HCO Policy Letter - Penalties For Lower Conditions)

www.ronthenut.org
Website about L. Ron Hubbard

www.lisamcpherson.org
Lisa McPherson died while in the care of Scientology - the autopsy found that she had over 100 cockroach bites on her body.

whyaretheydead.net
Scientology associated deaths

www.xenutv.com
Videos made by a protester

www.xenu.net
Infomation about Scientology - long established website

www.suppressiveperson.org
Website dedicated to defending "SPs" - they're not consigned to the history books yet.