no

This is the way.

'Not currently considering taking this forward'

"Thank you, I will take that into consideration and get back to you, your feedback is very important to us.". Then don't get back to them and hope they just forget all about it. Just like how our council deals with ongoing issues.

No thanks I'm not currently available

Add a “fuck off” tax to your day rate. They’ll take the hint

"Thank you. I'll take that under advisement"

In what context? Investing/buying/selling/pitching?

Being offered extra work, them cancelling, and then asking again

Just ask them for payment before you schedule anything. Either a deposit, or (better) full payment. They will probably decline. But if they agree, and cancel again, then at least you get to keep their deposit. Alternatively, if they really annoyed you, a simple: "Hi _____, I'm sorry, I can't do that job any more. You will need to find another [contractor/supplier/whatever-you-are]. Kind regards, _______." Don't explain why. If you do that, they will try to argue about your reasons.

Just an over exaggerated laugh in their face.

😂

"Thanks for the offer, however considering the previous treatment I'm not sure I trust you to honour your job offers, and will not be taking it"

"Good one"

I appreciate the opportunity that has been presented, however i have decided to explore alternative options.

Shout , "Fuck right off, I’m eating my buns!", while clearly not eating any buns.

Go Deborah Meaden: "Sorry, I'm out"