How to disabled Drupal builtin "poormanscron" cron when using Ultimate cron?
You don't need to uninstall any module. The setting you are looking for is on admin/config/system/cron/settings/poorman.


  


Change Launcher to -- Disabled -- from the default Serial, and Poormanscron should not run anymore.

If you disable or uninstall the Ultimate cron module, Drupal shows the equivalent settings in admin/config/system/cron, at the bottom of the page.


  


In this case, change Run cron every to Never.