how to delete OS

This sounds like Ransomeware or a very bad Malware that hopefully is only Malware. Follow the below directions to remove Malware / Adware Should the Ransomware message still appear - advise.

Using a Time Machine Backup to Revert to previous OS and was done while the Ransomware had control of the computer would nor solve the issue but only possibly re-introduce the Ransomeware back onto the computer.

Suggest downloading from a Trusted Developer and Respected ASC Contributor the application Malwarebytes for Mac. It is free or paid for added features. Run the Application and it should remove the malware / adware. Once done, restart computer and test.

There are no known Viruses in the wild that self replicate and affect macOS. There are Malware and Adware that does affect macOS and are often times downloaded as part of an Application from Third Party UnTrusted Site and get installed along with the Application.

What is not working in Big Sur that one wants to remove it. Has one attempted to work through the issues and resolve them ?

Suggest downloading the Application Etrecheck directly from a Trusted Developer and well Respected ASC Contributor. The application is free or paid from added features. Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ). It will take a Snap Shot -  both the hardware and software. The Report will Not Reveal Any Personal Information. Post back the Full Report - copy and paste - using the Additional Text Icon ( 3rd Icon to last )

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.

As a very last resort - Revert your Mac to a previous macOS version

Agree with previous person posting here.

We, on these ASC Forums, are just like yourself in that we are Volunteers with possible solutions, advise and out time.

Good Luck and really hope the " two local computer-security guys " really know how macOS works or the computer could very well be worst off after they get their Hands on it.

Further, are you Expecting to get Ransomeware ?

Boot into Recovery mode and erase the internal volume(s) or, better yet, boot into Internet Recovery mode or a flash installer and erase the whole internal device. But make backups before doing so.

Well then, why did you come here for advice?

Advise for an non-existent issue - come back when there is a real issue