Parents do not have a right to enter schools anytime they wish and to go anywhere they want. It is up to the district - or campus, if the district has delegated that authority - to set rules for public access to the campus and classrooms. Some schools have very open policies, encouraging parents to visit at anytime without advance notice, and others have very restrictive rules, allowing little to no classroom access except when specifically invited. Most schools fall in between.
Whatever policy on parent visitation is adopted, it should apply to all parents equally, but abuse of the policy or abusive, disruptive, or threatening behavior by a parent can result in special rules being made for that parent. In extreme cases, parents can be removed from campus and given no right of access at all.
A post from another blog last February talks about a parent who was formally banned from campus and then arrested when she ignored the ban and came onto the property. A number of commenters knew her personally and mention the apparently long and difficult history she had with the campus.
Banning parents is relatively rare, fortunately, but it can be done, and I find that in almost all the cases there has been a long and troubling history of severely inappropriat or threatening behavior by the parent.
So what should a classroom teacher who is having problems with a parent do?
Talk to your campus administrator. If the problem is fairly minor, let the principal know what is going on and ask for support for a policy that sets specific limits on the parents' visits, such as requiring advance notice, or limiting visits to once a week, or to 20 minutes at a time, or to certain times of day - whatever you believe will help the situation.
If the problem continues or is serious from the outset, ask the principal to consider giving the parent rules, or preventing the parent from making classroom visits at all. You will need the support of the principal to keep the parent from visiting, and only the principal can ban the parent completely from campus. Your job as a teacher is to make sure the principal has good, accurate information about the situation.