Instant Hotspot should only work for devices on the same account, for any other situation they then require the password which if you change on the hotspot device should stop those types of connections dead.
If the “other” device is not yours then the only option you could try is “Reset Network Settings” on your 6s Plus, that might remove whatever is allowing this to happen and therefore stop the “other” device connecting automatically.
The strange thing is this shouldn’t be possible, so you have either discovered a bug which you really need to inform Apple about or if the “other” device is yours could a setting of been transferred to it from via a Restore of a previous backup for example.
ps. Sorry didn’t see your recent post that you have now managed to fix, glad you got it sorted.