Spinel could've done this in Pokemon Horizon Episode 111...

[Dinoenjoyer2829]

Spinel didn't capture Liko, Roy, Uruto, and Amethio right there when they lost against Six Heroes and take Terapagos and White Zygarde to make sure they are no longer threat to the Exceed/Explorer in Horizon Episode 111. That is so mind-boggling since Spinel is supposed to be smarter and always take a perfect opportunity when there was a chance. He could've framed the Liko and co for attacking and attempt to steal Six Heroes Pokemon there and easily jail them. Or Spinel could use Liko and other to be forced to partakes in Laquium and Super Strong Sphere experiment and data collecting as well. It win/win for Spinel! I might consider making fanfiction of this actually but what do you think?

 

[NumberOneSpinelApologist]

There is a bunch of possible reasons:

1] it could be slightly too dark for the tone of the anime

2] Spinel already got all the data that he wanted from this experiment and he no longer needs them for this purpose, and why keep the lab rats in and have to deal with the costs for no reason? Disposal would be most efficient, but yet again, too dark.

3] there is a lot of possible gains for him for letting them roam free.

a) He is a scientist. If he did want more data about terapagos, after all of his run ins with it, he would realize by now that observing it in natural conditions will give him the best results. Field research about Pokémon behaviour can give you things that carefully engineered experiments will not. Most of the "interesting" things that Pagogo did were a surprise to him that happened sponteaneusly because it was put under stress.
He does not need them for laqurium experiments, he already has plenty of other, more willing and easier to control, subjects.

B) he gets political gains. As long as the volteccers are free he can blame whatever collateral damage he causes on their activities, while continuing to be a saviour in the eyes of the public

C) If he actually decided to sue them, it could cause problems- if he wanted to make sure that no one in the legal route believes the volteccers, he would have to bribe like, everyone, and this might be impractical. The volteccers do have dirt on him, obviously, so if they had a chance to speak in some sort of court this could sway the opinion of the public just a little bit.

 

[LegendsCelebi]

In addition to what the aptly named user wrote above:

Voltteccers are perfect "sparring partners" for Spinel and his increasingly deranged superpowered pokemon. The protags are highly capable trainers, with powerful pokemon. They know Spinel's tactics, know the strengths and weaknesses of the Six Heroes, they are aware of Laqurium and know how to plan around it. If Spinel can defeat them he can defeat anyone. But there's no value in battling Volteccers if they're not allowed to grow stronger, so Spinel let them go to train and pose a greater challenge whenever he decides it's time for another "live ammo exercise".

And on top of that Explorers vs. Volteccers are high-profile, high-powered battles which are completely invisible to the larger society. Yes, in theory Spinel could hire a bunch of top trainers for battling, or arrange "advertisement matches" with e.g. league champions to test his product agains the best of the best, but that would bring massive attention and scrutiny. And probably it wouldn't be good for Spinel to show off the Five Heroes in a battle with anyone who could tell a credible tale about them. Meanwhile Volteccers are "a bunch of troublemakers" in the public view, they can blabber what they want about a team of turbo-laqurium legendaries in Spinel's basement... No one will believe this.

 

[Le Lapin de Nelvana]

Spinel didn't capture Liko, Roy, Uruto, and Amethio right there when they lost against Six Heroes and take Terapagos and White Zygarde to make sure they are no longer threat to the Exceed/Explorer in Horizon Episode 111. That is so mind-boggling since Spinel is supposed to be smarter and always take a perfect opportunity when there was a chance. He could've framed the Liko and co for attacking and attempt to steal Six Heroes Pokemon there and easily jail them. Or Spinel could use Liko and other to be forced to partakes in Laquium and Super Strong Sphere experiment and data collecting as well. It win/win for Spinel! I might consider making fanfiction of this actually but what do you think?

and then once Spinel took the Black Rayquaza hostage from the Dragon Rampage Gang, he has finally made everyone use his "Drug" Spheres and has everyone bow down to him like a demigod