Kick to the nuts is legit
I disagree with anyone saying "just tease/play with him", it isn't being helpful. If you feel like self-defense is needed, then messing around and embarrassing the antagonist is not an option in the scenario.
In other words, if you are in a situation where toying around would work, then you are so overmatched to the other guy/group that they would have to basically be invalid or so cowardly and weak as to not be ANY kind of threat in the first place. And even small skinny people can easily pose a decent threat if they rise to it, so that excludes basically anyone (it's not realistic to be able to just "play" with any kind of actual threat)
If some rube is denying your rights to exist in peace, a nut kick (or a bouquet of) is fully warranted. It's not even that bad because it happens by accident to everyone in all types of regular sports and mere everyday mishaps. Plus, they'll be fine afterward, unlike most other things that it would take to stop them once they go aggressive
There is no honor among unsolicited assaulters. Why grant them the upper hand to better enable victimizing innocents when they don't even respect you for it? They need their lesson, do your duty and give it to them
Cliffs: If you stubbornly must "play" with an attacker, you can "playfully" kick them in the shorts