The "activists" are as free to protest this billboard as its renter is to post his views, however "unpleasant" or "confusing," as one of the activists called them, they may be.* No state action here, and to claim it would be a "hate crime" would stretch the limits of even that fluid term, though any vandalism to the billboard would clearly be a crime. This is as good an example of ideas competing in the marketplace as any. As for the billboard owner's "not being interested in the activists' business," however, that might run afoul of any anti-discrimination laws Ohio may have.
*Memo to "activists:" free speech on hot issues is often "unpleasant" - talk about a puss-ass word! - and I've enough faith in most of my fellow citizens to be able to form judgments w/o being "confused," TYVM.