Completely reset Mojave disk
Doing a clean install over the Internet

Hold down the Option+⌘+R keys at startup to boot to internet recovery. After booting to recovery mode, use the Disk Utility to erase the drive. Choose the APFS format. 

Doing a clean install using a flash drive

You can also choose to download Mojave from the App Store. You can then transfer the installer to a flash drive. Afterwards, restart the Mac and hold down the Option key until the Startup Manager icons appear. Choose to boot from the flash drive. Use the Disk Utility to erase the drive. Choose the APFS format.