No, Not Again. Thanks.

I have been wracking my brain.  What is the best way to tell a guy "not in this lifetime, not interested, never going to be?"  I have heard:

• Ignore.  Do not respond to texts or pick up calls.  Throw all emails into spam
• Say "There is no chemistry"
• Say "It's not you, it's me"
• Say "I am not ready"
• Be incredibly busy

He's nice and smart, I would have to be drugged for anything to happen between us.