Programmatically stop Permissions inheritance from sharepoint list

You can stop the security inheritance of a Web site, list, or list item through the BreakRoleInheritance method of the object so that role assignments on the parent object no longer apply to the child object.

An example,

using System;
using Microsoft.SharePoint.Client;
using SP = Microsoft.SharePoint.Client;

namespace Microsoft.SDK.SharePointServices.Samples
    class BreakSecurityInheritance
        static void Main()
            string siteUrl = "http://MyServer/sites/MySiteCollection";
ClientContext oContext = new ClientContext(siteUrl);
            SP.List oList = oContext.Web.Lists.GetByTitle("Announcements");

oList.BreakRoleInheritance(true, false);