You need to tag these users who started this AI flow in some way, it could be with a TAG or the current date in a custom field. 
And add a condition at the beginning of this flow, so that users who start this flow, if the condition is met, nothing will be executed.

Hi ​@josuealfonso you can set it up as ​@rogerioaraujo mentioned, with tags. It looks something like this:

After the AI step you’d tag the person, and before it you’d have a condition to make sure they go through the AI step only once.

You could also set your Default Reply to trigger only once every 24 hours.

Depending on your scenario this might be the easiest solution for you.

Thank you for your response. The situation is that if I add the conditional value as the first step of the flow, right after the trigger, the message appears: Choose the first message for the Messenger channel to set up Facebook ads. So, I believe it’s not possible to start the default flow with a condition, only with a message. Or maybe there's something I’m not understanding correctly.

Hi ​@josuealfonso you can set it up as ​@rogerioaraujo mentioned, with tags. It looks something like this:

 

After the AI step you’d tag the person, and before it you’d have a condition to make sure they go through the AI step only once.

You could also set your Default Reply to trigger only once every 24 hours.

 

Depending on your scenario this might be the easiest solution for you.

Thank you for your response. The situation is that if I add the conditional value as the first step of the flow, right after the trigger, the message appears: Choose the first message for the Messenger channel to set up Facebook ads. So, I believe it’s not possible to start the default flow with a condition, only with a message. Or maybe there's something I’m not understanding correctly.

​@josuealfonso if you're using the Ads trigger, then you won't be able to do much logic before the first message (since this is limited by Meta).

But you can add logic after that! So you'll have to play with the default reply and see exactly what the user flow will be, so you can design the automation the way you intend it to work.