Yes and no
with this stuff we get into the murky areas of slander & libel. I cannot go fire up a website and proclaim to the world that someone I don't like is a Neo-Nazi or a child-molester.
But it gets trickier with public figures, or if the speech is considered parody, etc.
In the case of Ayaan Hirsi Ali, she is a semi-public figure, but not a politician or political leader, and the SPLC's designation of her as an "Anti-Muslim Extremist" was both wrong, and based on vague and capricious criteria (there is no science behind any of it--it is just a hit piece). The piece was not a parody. She could have sued.
Nawaz DID sue the SPLC and won 3.3 million
The Southern Poverty Law Center (SPLC) has been hit with a big payout after losing in a defamation lawsuit to Quilliam International and Maajid Nawaz, the founder of the organization. Nawaz objected to being mentioned in SPLC's "Field Guide to Anti-Muslim Extremists" and sued for defamation.
lawandcrime.com
And just like the Devil mixes lies with the truth, the SPLC's critique of the FRC is partly accurate, and partly false. They claim that the statement from the FRC that “The reality is, homosexuals have entered the Scouts in the past for predatory purposes," is an attack on all LGBT people, when it is simply a factual claim: the scouts have had legal problems in the past due to child molesters.
The SPLC lives in that cesspool level of the Internet, and operates a grift by scaring Americans into giving them money in order to root out "extremists" --it is a hyper-partisan propaganda outlet.