Add wait(1) before ent_remove should work :

Code:

---

function removeRock()
{
   if (event_type==event_entity)
   {
      wait(1);
      ent_remove(my);
   }
}

---

Also, make sure the rock is moved using c_move function
and enable_entity is turned on :

Code:

---

action moveRock
{
   my.enable_entity = on;
   my.event = removeRock;
   while(1)
   {
       c_move(.....);
       wait(1);
   }
}

---

If it still doesn't work, post your "rock" action codes here,
otherwise we never know what's your problem.