If you are renting say a trailer in a trailer park and you have friends come over to visit does the landlord have the right to kick the visitor out of the park and ban him from returning to visit someone that lives in the park just because he feels like it

You don't say what State you live in, but each state has what is known as a LandLord/Tenant Act. It states what a Landlord and a Tenant can and cannot legally do relating to the use of, the care/occupancy, repairs, payments, evictions, right to enter the property, among other things.

You may need to review the terms of your lease and see if there is anything that gives your landlord permission to do what you state they are doing. If you don't have a formal lease, then you still may be covered by the Landlord/Tenant Act of the State you reside in. This is all just very basic information and it may be in your best interest to learn why the landlord has banned someone or would even want to.
Good luck.

Not just because they feel like it, but for example apartment complexes and trailer parks can ban ( give criminal tresspass warnings) to people who are causing problems, using drugs or more.

The standard boiler plate lease in a mobile home park states that you cannot have guests stay for more than one week (some cases 14 days) without registering the person with the park office as living in your home. The landlord or park manager can make the extra person leave the premises. You need to read your lease or park rule book to see exactly what the rules are regarding guests that stay overnight.

If your friends are just coming over to your house to visit, your landlord may not wish to have all the traffic coming onto the premises which could mean drug traffic in their mind. Also, if the mobile home park is considered private property, the landlord could ban the person from the property. It's his property, not yours, you just rent the space where the trailer is located. Got to play by their rules or move the home.