How do I uninstall Plesk on Ubuntu 16.04?

Hello there,

It is recommended to rebuild your server, as removing Plesk could cause corruption of certain services.

If you still wish to remove Plesk from your server, some services may not operate as intended. Because of this, use this removal script at your own risk.

In order to uninstall Plesk, you will need to download a script to do so. You can do this by issuing the following commands:
To download the script:
wget https://support.plesk.com/hc/articleattachments/115004353885/pleskremove.zip
To unzip, grant execution permission, and execute:

unzip plesk_remove.zip

chmod +x plesk_remove.sh

sh ./plesk_remove.sh

https://support.plesk.com/hc/en-us/articles/213907125-How-to-uninstall-Plesk-for-Linux

I hope this helps point you in the right direction.