door to door sales people

hello....i have had two incidences of door to door sales people for uverse knock on my door even though i have a clearly visible sign on my door saying no solicitors.  this happened for the second time today.  two men walked up to my door an knocked, disregarding the sign.  i answered from my window and he argued with me about opening my door.  i clearly stated how i would not open my door for men i do not know so they decided to laugh at me and make fun of me.  super classy.  how do i stop this from happening even though i have it clearly stated they are NOT to knock on my door??