You can get & should post some signs at the front edge of your yard or where ever they are entering or leaving their "business". You do have to be clear you do not like or want the action.
I do think the ones saying, " Yard has been spayed= KEEP OFF!" or "Insecticides have been applied, Keep Off!" - would be the best, and most likely to WORK - as intended. (I had some made.) There are also plenty of signs clearly indicating (by visual signage or wording - on eBay and Amazon) that you do NOT want dogs POOPING on your lawn.
I do think using a camera to determine WHO and/or which dogs are doing this WILL be necessary. These can be RENTED - if you do not want to invest. You then will need to send a letter to the owners about NOT using YOUR yard as their dog's potty stop, OR that they need to be PREPARED if an "accident" occurs, by carrying & using ziploc baggies to pick it up & taking it AWAY. Do not threaten to retaliate but see if there any laws on your town's books. If not, you either start RETURNING the poop - if it still happens or have a large LOAD of cow or horse manure dumped on their lawn (if you dare.... & can pay for it in cash). <LOL>
"Get OFF MY GARDEN" is a noxious (to dogs) gel you can apply, which can help DETER dogs from coming INTO your yard, but has to be reapplied, after a rain. Cayenne pepper is fairly cheap, bought in Latino grocery stores in larger containers & can be very irksome to dogs or cats if used on the perimeter, but again can be easily washed away.
Getting motion -sprinklers (often used to deter DEER is the best advice, IMO) will keep off OWNERS as well as dogs & cats but cannot be used in freezing temperatures. Great for spring, summer & early fall though!!!