How to disable dietpi postbooot ?

I want to disable large banner shown on pi terminal after I ssh to it , it covers all the space as I login from iphone.

First of all you can reduce the banner size by running dietpi-banner and e.g. deselecting “Useful DietPi commands” and other entries you do not require.

You can as well disable dietpi-postboot via: systemctl disable dietpi-postboot
But note that then no update check is done on boot and DietPi-controlled services are not started. So you should then run dietpi-services and switch all services to systemd-controlled, so that they are started on boot by systemd, usually on earlier boot stage then.