Stop sharing between iPhones

define "try to exchange data".

What exactly are they doing or trying to do?

With iOS 17, when the iPhones are together they will activate a connection, but that does not mean they are actively trying to exchange data. Just that they are connected to each other, because they are in very close proximity.

At worst they'll exchange contact info, if they don't already have it.

Again, they are not transferring anything beyond a single contact for themselves if they don't already have it. If they already have it, nothing gets transferred.

You can however turn off the feature in Settings ➜ General ➜ AirDrop ➜ Bringing Devices Together and switch it off.