You can easily make a program unkillable (or to be more precise untermable) on Linux. Here’s a simple bash script that will do that.

#!/bin/bash                                                                                                     function finish {
  while true
  do                                                              
    echo "Can't kill me."                                   
    sleep 10                                        
  done                                            
}                                                                                                               trap finish EXIT                                        
trap finish TERM                                        
trap finish INT                                                                                                 

while true                                              
do                                                              
  echo "Still alive."                                     
  sleep 10                                        
done