Hey there! This question has been coming in quite a bit lately.
Right now, if you're seeing an influx of subscribers you weren't expecting, it might be because Facebook featured your bot on a Messenger homepage somewhere.
We checked in with our people on Facebook, and it looks like there's no way to control which bots get featured at this time :(
Don't worry though - they told us they're working on something which will let you opt-out of the bot auto-feature.
That way, you can have way more control over who has access to your bot, and you won't have to be charged for subscribers you don't want!
We are waiting for this feature to be deployed from Facebook.
We'll keep you in the loop!