this is an easy fix...it's called negotiation. (courts call this
arbitration)
If the fella won't see your point of view about "I didn't ask you to do
all this stuff you're charging me for" then you tell him.."ok, here's a
check for the amount you originally quoted."

Write "payment in full, services rendered to chimney on month/day/year"
If he cashes this he is acknowleging that you are not obligated to
make further payments. This will hold up in court. (doesn't do so
well if you do it on a car payment, i'd imagine ;-) )

I write that into the memo of anything and everything I have services
done for... I write checks like that to my dentist, doctor, optomitrist
as well.

Honestly though, if this guy isn't a quack he'll probably entertain
your offer of negotiation. Use the analogy "If the neighbor kid came
over and raked my leaves without asking first, I wouldn't feel it was
right to pay him either"