there are some possible theories under anti-trust law (Federal Trade Commission prosecutions including in some instances organized plots to withhold patronage or advertising, also of course DOJ AntiTrust Division), and the RICO act has some interesting possibilities. (Plus several states have laws that may also apply.) More statutes could come into play. It would all depend on the nature of the available evidence, there are some rather fine distinctions in some cases that would have to be examined ...
That’s for starters, of course there’s always the good old common law conspiracy doctrine, too...
“(Federal Trade Commission prosecutions including in some instances organized plots to withhold patronage or advertising, also of course DOJ AntiTrust Division)”
Except for Obamacare, there are no federal laws that require a person to patronize a certain business or for a company to buy advertising on a particular media outlet. Boycotts are legal and we are all free to boycott a business.