FYI it’s possible to put measures on the ballot in 24 states banning private prisons, including 3 of the 5 states with the largest private prison populations. Avg cost per state to put it on the ballot would be $1 million.
Here are the states that allow ballot initiatives. The blue, green, and orange states can ban private prisons via ballot initiative. ballotpedia.org/States_with_in… 
And we also know how much it costs, on average, to fund a signature gathering effort that successfully puts a measure on the ballot. In 2016 it was $1 million. ballotpedia.org/Ballot_measure…
Note it could be *less* than $1 million if enough volunteers are recruited to handle a large portion of the signature collection. On such an issue, recruiting volunteers who care about this issue would be relatively easy.
Voters would have to approve these initiatives and we don’t know for sure what they’ll do (haven’t seen polling on private prisons). But the public is generally supportive of criminal justice reform & private prisons are an obvious part of it. My guess is the measures would pass.
Note this would work for a range of issues - changing police deadly force standards, expanding Medicaid, protecting abortion rights. Just takes grassroots enthusiasm and some funding (tho probably less than would be spent lobbying/organizing to pass these in state legislatures).
Also note that this would ban private prisons at the state level but not at the federal level. However, 73% of all people incarcerated in private prisons are incarcerated at the state level so this is where most of the change has to happen anyway. sentencingproject.org/publications/p…
