In effect, Bristling Boar can't be blocked. Bristling Boar can't be blocked except by two or more creatures due to the menace counter (C.R. 702.110b, 122.1b), but it also can't be blocked by more than one creature (that is, two or more creatures); the two restrictions combine so that Bristling Boar effectively can't be blocked (C.R. 509.1b; see also C.R. 101.1, 101.2). See also this thread.
EDIT (Sep. 30): Add rule citation.