My neighbor?s dog is always coming over to my gravel driveway to do
his business (shit).  I need to find a way of preventing him from
doing that without setting up fences.  One idea I have is to find a
non-toxic chemical to put down on the driveway that will either smell
bad to the dog but not to humans or irritate his paws enough to make
him want to stay away but not do any serious harm to him.  I need a
chemical that would not kill my walnut tree or any of the shrubs in my
garden that might be affected by water run off after it rains.

I would like a list of 5 such chemicals that could serve for this
purpose that would not cost more than US$30 to buy.

Can anyone help me with this?

I think I can help.

"Boundary Repellent - 14 oz Aerosol   
A training aid to keep animals away by spraying forbidden areas and
objects as directed. For indoor and outdoor use. Directions: Spray
area attracted by pet daily until habit has been changed. Read
directions thoroughly since some items should not be sprayed on."
6.99 from KVVet.com
http://www.kvvet.com/KVVet/productr.asp?pf%5Fid=54601&gift=False&0=dept%2Easp%2Cdept%5Fid%3D198%26Tree%3D%2CComplete%20Catalog&1=dept%2Easp%2Cdept%5Fid%3D123%26Tree%3D0%2CPet&2=dept%2Easp%2Cdept%5Fid%3D39%26menu%5Fid%3D%26Tree%3D1%2CRepellents&3=dept%2Easp%2Cdept%5Fid%3D514%26menu%5Fid%3D%26Tree%3D2%2CAnimal%20Deterrents%20%26%20Bandage%20Protection&4=product%5Ffamily%2Easp%2Cfamily%5Fid%3D438%26T
ree%3D3%2CBoundary%20Repellent&HSLB=False&mscssid=2C5E7395A2314F7F9E1D49B1AE16DA4D

"Indoor/Outdoor Repellent - 14 oz
Keeps pets and stray animals away from forbidden areas including
furniture, rugs, gardens, and more. Helps change undesirable habits.
For cats and dogs."
$5.95 from KVVet.com
http://www.kvvet.com/KVVet/productr.asp?pf%5Fid=50112&gift=False&0=dept%2Easp%2Cdept%5Fid%3D198%26Tree%3D%2CComplete%20Catalog&1=dept%2Easp%2Cdept%5Fid%3D123%26Tree%3D0%2CPet&2=dept%2Easp%2Cdept%5Fid%3D39%26menu%5Fid%3D%26Tree%3D1%2CRepellents&3=dept%2Easp%2Cdept%5Fid%3D514%26menu%5Fid%3D%26Tree%3D2%2CAnimal%20Deterrents%20%26%20Bandage%20Protection&HSLB=False&mscssid=2C5E7395A2314F7F9E1D49B1AE16D
A4D

"Outdoor Repellent - 32oz   
Repels animals from shrubs and plants. Contains 5 bitter herbs. Use
Outdoor Repellent to STOP dogs, cats, rodents, rabbits and deer from
chewing up your property! Use on Trees, shrubs, potted plants, flower
beds, patio furniture, fences and siding, garbage cans/sacks and
more."
$7.59 from KVVet.com
http://www.kvvet.com/KVVet/productr.asp?pf%5Fid=54609&gift=False&0=dept%2Easp%2Cdept%5Fid%3D198%26Tree%3D%2CComplete%20Catalog&1=dept%2Easp%2Cdept%5Fid%3D123%26Tree%3D0%2CPet&2=dept%2Easp%2Cdept%5Fid%3D39%26menu%5Fid%3D%26Tree%3D1%2CRepellents&3=dept%2Easp%2Cdept%5Fid%3D514%26menu%5Fid%3D%26Tree%3D2%2CAnimal%20Deterrents%20%26%20Bandage%20Protection&HSLB=False&mscssid=2C5E7395A2314F7F9E1D49B1AE16D
A4D

"Repel - 16 oz Aerosol
America's best selling line of animal repellents are used to prevent
defecation and urination on lawns and gardens, also cats digging in
gardens and other areas. Unique formula causes nasal discomfort to the
intruder."
$5.99 from KVVet.com
http://www.kvvet.com/KVVet/productr.asp?pf%5Fid=58810&gift=False&0=dept%2Easp%2Cdept%5Fid%3D198%26Tree%3D%2CComplete%20Catalog&1=dept%2Easp%2Cdept%5Fid%3D123%26Tree%3D0%2CPet&2=dept%2Easp%2Cdept%5Fid%3D39%26menu%5Fid%3D%26Tree%3D1%2CRepellents&3=dept%2Easp%2Cdept%5Fid%3D514%26menu%5Fid%3D%26Tree%3D2%2CAnimal%20Deterrents%20%26%20Bandage%20Protection&HSLB=False&mscssid=2C5E7395A2314F7F9E1D49B1AE16D
A4D

Liquid Fence Dog and Cat Repellent
"Liquid FenceŽ Dog & Cat Repellent keeps your pets, your neighbor?s
pets and strays away from areas they don?t belong - lawns, flower beds
and garden areas."
$23.99/gallon
http://www.liquidfence.com/index.asp?PageAction=VIEWCATS&Category=203

Also, for really persistent animals who don't mind chemicals, there
are motion-sensor sprayers:
"Scarecrow Motion Activated Sprinkler   
The Scarecrow is a motion activated sprinkler that chases animals away."
http://www2.yardiac.com/long.asp?tgs=14357468:29976472&cart_id=&item_id=929
But it's expensive, around $60.

It was simple question but the answer was bang on and very thorough.  Good work.