There is no option to exclude the QR code from the welcome email.
If you want to stop mobiles being used, you could always tick 'Block Remote Tunnel Connections (3CX Client connections with Tunnel enabled & SBC will be blocked)' under Options tab for the extension.
Ok thank you for you answer.
I know but I can't block remote connections because a lot of softphones use this option.`
I try to block only the smartphone apps.
I would recommend posting this as a feature request to our ideas section of the forum https://www.3cx.com/community/forums/3cx-ideas/ . All posts are reviewed by their corresponding Departments and, if feasible and voted for by other users as well, may be implemented in an upcoming service pack or update