C# sample code to deny TerminateProcess rights to non-admin peers.

Immortal.cs

using System;
using System.Security.AccessControl;
using System.Security.Principal;
using System.Diagnostics;
using System.Runtime.InteropServices;

class Program
{
    public class ProcessSecurity : NativeObjectSecurity {
	    public ProcessSecurity(SafeHandle processHandle)
	        : base(false, ResourceType.KernelObject, processHandle, AccessControlSections.Access) {}
	
	    public void AddAccessRule(ProcessAccessRule rule)
	    {
	        base.AddAccessRule(rule);
	    }
	
	    public void SaveChanges(SafeHandle processHandle)
	    {
	        Persist(processHandle, AccessControlSections.Access);
	    }
	
	    public override Type AccessRightType
	    {
	        get { return typeof(ProcessAccessRights); }
	    }
	
	    public override AccessRule AccessRuleFactory(System.Security.Principal.IdentityReference identityReference, int accessMask, bool isInherited, InheritanceFlags inheritanceFlags, PropagationFlags propagationFlags, AccessControlType type)
	    {
	        return new ProcessAccessRule(identityReference, (ProcessAccessRights)accessMask, isInherited, inheritanceFlags, propagationFlags, type);
	    }
	
	    public override Type AccessRuleType {
	        get { return typeof(ProcessAccessRule); }
	    }
	
	    public override AuditRule AuditRuleFactory(System.Security.Principal.IdentityReference identityReference, int accessMask, bool isInherited, InheritanceFlags inheritanceFlags, 
	    											PropagationFlags propagationFlags, AuditFlags flags) {
	        throw new NotImplementedException();
	    }
	
	    public override Type AuditRuleType {
	        get { throw new NotImplementedException(); }
	    }
	}

	public class ProcessAccessRule : AccessRule
	{
	    public ProcessAccessRule(IdentityReference identityReference, ProcessAccessRights accessMask, bool isInherited, InheritanceFlags inheritanceFlags, PropagationFlags propagationFlags, AccessControlType type)
	        : base(identityReference, (int)accessMask, isInherited, inheritanceFlags, propagationFlags, type) { }

    	public ProcessAccessRights ProcessAccessRights { get { return (ProcessAccessRights)AccessMask; } }
	}
	[Flags]
	public enum ProcessAccessRights
	{
	    Terminate = 1
	}

    static void Main(string[] args)
    {
        var hCurrentProcess = Process.GetCurrentProcess().SafeHandle;
        var processSecurity = new ProcessSecurity(hCurrentProcess);
        SecurityIdentifier sid = new SecurityIdentifier(WellKnownSidType.WorldSid, null);
        // Create a rule to deny process termination.
        ProcessAccessRule rule = new ProcessAccessRule(sid, ProcessAccessRights.Terminate, false, 
        												InheritanceFlags.None, PropagationFlags.None, AccessControlType.Deny);
        processSecurity.AddAccessRule(rule);
        processSecurity.SaveChanges(hCurrentProcess);

        // Keep the process running until the user hits a key.
        Console.WriteLine("Press any key to exit... \n(Try terminating via taskkill.exe, observe it doesn't work unless you run as administrator.)");
        Console.ReadKey();
    }
}