Can the host stop participants from using AI Companion?

When making a meeting, go to Options > Show and then see the respective settings for Automatically start meeting summary/questions. These two-settings are inherited setting from the host's user settings, but you can toggle them for each meeting regardless. When these settings are off, the host needs to intentionally allow the participants access to AI features, and they have to ability to select which meeting roles have access.

For summaries, only the host gets to decide whether summaries are generated or not, even if a participant has enabled this feature for their own meetings. For asking questions to the AI, the host can select to enable or disable the feature when setting up a meeting, and the host is also able to only enable the feature for themself.

Hope this helps.

I saw these options. To confirm, if the host has meeting summaries turned off, a participant may request that the host turn the feature on. Still, an individual participant (i.e., not a host or co-host) cannot turn it on as a type of local action -- such as a local recording. 

I'm unclear what you mean by "even if the participant has enabled this feature for their own meetings." "Their own meeting" seems to mean being the host. Or are you saying that the participant may have AI features turned on in their account, but that does not mean they are turned on for other people's meetings?