Not sure of a “defense”, although you would be entitled to it, I hate seeing an animal suffer (even pepper spray or mace) because the owner is stupid (sort of like, “there is no such thing as an illegitimate child, simply one [or more commonly two] recklessly irresponsible quasi-adults.”). Spraying the OWNER although tempting and certainly poetic justice is probably not advisable. SPAYING the owner, preferably before they procreate, likewise tempting but in most cases not practical.
You may want to try out some of those pocket pen spy cameras, I don’t know how well they work, and I am sure there is a lot of variation in quality, but you could surreptitiously turn it on when uninvited dogs or people approach you (may want to check your local laws on this). AFTER turning it on as they, without your permission, approach, THEN get out your real phone or camera and tell the owner that you are about to take pictures to document violation of park rules (BTW, make sure your rocket launching is okay, first! Sorta like, You don’t go into a police station to report a theft carrying a loaded weapon without a permit.)
The reason for the first camera is some ill mannered people (there are other terms but this is a family forum) may threaten you if you try to film them openly. Assuming surreptitious video is legal under these circumstances, their actions may dig them a deeper hole.
The other good thing about having a body-cam on you that YOU know is documenting things is YOU are more likely to stay calm and reasonable. Ideally when you pull out your phone or camera the violator will restrain his or her dog and back off, if not you want clear continuous unedited audio and video footage that shows you being calm and rationale and the dog owner, well, not.
I also think a really good source of information (although I also believe in the tooth fairy) would be to contact your local police station and ask them how a civilian should handle such a situation. They should also be able to provide input of the legality of both announced and surreptitious video without the other’s consent. I don’t want to get YOU in trouble!