csrutil disable
Restart computer
In the terminal, type
sudo open /Applications/TextEdit.app /System/Library/LaunchDaemons/com.apple.ManagedClient.enroll.plist
change
<key>com.apple.ManagedClient.enroll</key>
<true/>
to
<key>com.apple.ManagedClient.enroll</key>
<false/>
So that the changes take effect
I have no experience with iRemove.tools. Since I'm more of low level "do it yourself" tech, I have not investigated or tried other potential options. I am not interested in testing it because this is not a revenue generating model for me. I looked over the instructions and it looks like it just does the same thing posted in this thread. Basically wipe the Mac and disable network access. Then it wants to disable SIP so maybe it's installing its own firewalling/filtering driver or modifying other system files. Since I don't have a problem with the 1 second-hand Mac that has this MDM Enrollment problem, I will probably not try to reverse engineer what it is doing.
I think there was only 1 review where someone said it worked for them; perhaps the payment is enough for normal users who are not IT trained to work in CLI or with low level OS design, security, or programming methods.