I like to dispose of unwanted yukkuri by charring them in the microwave before dropping them back off with their families.
They seem to live substantially longer if you feed them, but once they're burnt enough it's only a matter of time. Also, once they're charred black they stop moving and talking, but still still board a sui if given and will still make eating noises.
If you are looking for some rare yukkuri in forest, you can kill every one you don't need. As soon as they die, new yukkuri will appear near you. Game has constant maximum of yukkuris in the forest near you. So you have to walk in the forest to find someone or kill everyone near you.
Google Play or a few other places offering Android apps. I think I've seen mention of a PC version. I don't have a link to it, but if you look in the forum in the Yukkuri Breeding thread or the Google Play page, there's a link to Bluesnow's site.