Sealioning is related to the Gish Gallop, to which there are some countermeasures that work (beyond systematically flagging his post every time he does it, which is the least we all can do to protect the community).

The approach that worked for me, and shut him up cold in his steps and made him suddenly retreat, is to be very calm and structured about your replies, and not let him derail you, which is something that we can all cooperate together on, and support each other by doing together as a tag team.

Carefully chose and enumerate your questions, and repeat them when he doesn't answer them, but don't get distracted by his off-topic diversions, or his passive aggressive belligerence with a veneer of sincerity on top. His pseudo-intellectualism and pretend-open-mindedness and saccharine-sincerity is all just an act, and you don't owe him the level of courtesy that he is pretending to extend to you and mocking you with. I can practically see him laughing to himself (and his Gamergate veteran buddies) behind the keyboard every time he thinks he's fooled anyone into taking him seriously, because he certainly doesn't believe what he says.

Just calmly point out every time he ignores one of your questions or somebody else's questions, or tries to derail the conversation. Keep relentlessly pushing him back onto the topic, and never let him derail you. When I did that, he was mortified and childishly complained that I was not the boss of him, and that I could not tell him what to do, or force him to answer my questions, then he immediately lost interest and fled without another word, because the last thing he actually wanted to to was to stay on topic and answer the questions I would not stop asking.

https://en.wikipedia.org/wiki/Gish_gallop

Technique and counter measures

During a Gish gallop, a debater confronts an opponent with a rapid series of many specious arguments, half-truths, and misrepresentations in a short space of time, which makes it impossible for the opponent to refute all of them within the format of a formal debate.[3][4] In practice, each point raised by the "Gish galloper" takes considerably more time to refute or fact-check than it did to state in the first place.[5] The technique wastes an opponent's time and may cast doubt on the opponent's debating ability for an audience unfamiliar with the technique, especially if no independent fact-checking is involved[6] or if the audience has limited knowledge of the topics.

Generally, it is more difficult to use the Gish gallop in a structured debate than in a free-form one.[7] If a debater is familiar with an opponent who is known to use the Gish gallop, the technique may be countered by pre-empting and refuting the opponent's commonly used arguments first, before the opponent has an opportunity to launch into a Gish gallop.[8]