I'm not suggesting you send invalid certificates. That would be silly. I'm providing information about how to identify invalid certificates and (indirectly) ways to tell teachers (politely) that their certificate isn't real.
When you click the activity and look at the list of issued certificates, invalid certs don't show up.
I've never used the validation code thing...but I bet invalid certs don't have one. You might test that and implement it.
Is there a good reason for them to have Teacher permissions in the course? Maybe take them away in courses where they are supposed to get credit vs. courses they teach. Of, if that doesn't work, remove the receive/view permissions from teacher role. Then they have to switch to student role to get the cert--so they can't get it without meeting the requirements.