Pornography, Public Computers, & Library Policy

Pornography and public computers in libraries have gone hand-in-hand for some time and I'm doing some research on how library policies should handle addressing this in a realistic and proactive way. The question that I am researching is whether or not it is legal to explicitly list pornography as something that cannot be accessed on library computers. I understand ALA and the Intellectual Freedom Committees stance on this issue as well as the first amendment ramifications and I am certainly not advocating for censorship, however, I've seen several policies that have tried to circumvent the issue by having vague, unhelpful policies and others that have flat out said that it is not allowed. Any clarifying help from a legal standpoint would be appreciated. 

My understanding is that it would not be constitutional to have a policy restricting pornography, however, there could be something in the policy that restricts the displaying of pornography or other offensive content. 

Thank you.