You can disable Smart Reply from the mobile apps, the setting isn't there on Web yet. Honestly we just haven't prioritized it yet.
Here's some info on how it works if it helps address your concern: We've transformed the language generation problem into a labelling problem. From a systems point of view it's not any different from spam filtering, or tabbed inbox, and we use the exact same features. Smart Reply uses a whitelist of around 30k independently generated & sanitized phrases. You could think of each whitelist entry as a label. So when an email comes in on the delivery pipeline we just label things, like we've always done. We label (spam/not-spam) for spam filtering, we label (promo/social/update/forum) for tabbed inbox, and we label (Thanks!/Sounds good!/etc/etc) for smart reply.
Here's some info on how it works if it helps address your concern: We've transformed the language generation problem into a labelling problem. From a systems point of view it's not any different from spam filtering, or tabbed inbox, and we use the exact same features. Smart Reply uses a whitelist of around 30k independently generated & sanitized phrases. You could think of each whitelist entry as a label. So when an email comes in on the delivery pipeline we just label things, like we've always done. We label (spam/not-spam) for spam filtering, we label (promo/social/update/forum) for tabbed inbox, and we label (Thanks!/Sounds good!/etc/etc) for smart reply.