is there a product to use on my front lawn to stop dog walkers dogs from defacating on my lawn??

It's bad that we have to even take any measures to have this not happen!  What happen to common courtesy?... I have a gated yard now, but when I did not, I used a product that a girlfriend tipped me to called Liquid Fence.  It works!  And so better than the "keep your dogs off my grass" sign! You should be able to pick it up at your local pet store, but if you cannot find it there, you can order it online. It's not only great for droppings, but also for strays that like to come and just lang out, leaving their lovely fleas!:

• All natural - Made from natural plant oils 
• Safe for your plants and family - Environmentally safe and biodegradable 
• Easy-to-use - Just spray and your problems go away  
• Great training aid 
• Worry-free dog and cat protection - Won't harm the animals  
• Proven effective at repelling dogs and cats - Member tested and recommended by the National Home Gardening Club!

There is also Repel From My Garden and Havahart Get Away 5402 dog & cat repellents you might also find work . Good luck and enjoy the rest of your wekend!