SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/for-developers-delete-page-permissions-for-users

ResetPermissionValue.cs

using System.Linq;
using Telerik.Sitefinity.Modules.Pages;
using Telerik.Sitefinity.Pages.Model;
using Telerik.Sitefinity.Security;
using Telerik.Sitefinity.Security.Model;

namespace SitefinityWebApp
{
    public class ResetPermissionValue
    {
        private void ResetPagePermission(string username, string pageTitle)
        {
            UserManager usermanager = UserManager.GetManager();
            User user = usermanager.GetUser(username);

            PageManager pagemanager = PageManager.GetManager();
            PageNode mypage = pagemanager.
                GetPageNodes().
                FirstOrDefault(pn => pn.Title == pageTitle);
            if (mypage != null)
            {
                Permission p = pagemanager.GetPermission(
                    SecurityConstants.Sets.Pages.SetName,
                    mypage.Id,
                    user.Id);
                if (p != null)
                {
                    p.Grant = 0;
                    p.Deny = 0;
                    pagemanager.SaveChanges();
                }
            }
        }
    }
}