I've got a long time (18 years!) employee that's broken an HR policy a couple of times. Employee Relations/HR has had more identical complaints since my last conversation with this employee (which resulted in a write-up stating that they would be terminated on next offense) and let me know that I would need to terminate the employee today.
I've never fired anyone before-- I'm about 3 years in to being a manager. This person is a good employee and performs well, but has issues with one particular policy. I am following the work procedures to get the termination done, which will be happening this afternoon. This person will cry during the termination. I'm not looking forward to it.
Any advice going in?

You're not firing them...
This person is choosing to leave.
From your description, this situation was out of your hands and the offender was given numerous chances to alter their behavior.
I do not envy you and I am sure it is a shame to lose a good performer.
If you feel the policy is just and would fire others who were continual violators, then your conscience is clear.
Good luck and use this time as a learning experience should you be forced to fire someone again in the future.
Appreciate the post
Thanks. It is definitely something that is out of my hands and this policy is not related to performance and I'd fire anyone for breaking it (especially for continuing to do it after I told them two months ago that another offense would result in termination). Clear conscience but still not the greatest way to start a week. Sad situation but I did what I could.
My game plan is to make it a short conversation, explain what is happening and why, and let the employee know that any further discussion or inquiry needs to go through employee relations/HR. I'll update in an hour or so (will be doing it at about 4pm est).
been there, done that
And it was NOT fun.
It sounds like they had a chance, and blew it.
HR can be an essential resource right now--let them help and make sure the person leaving is clear about the next steps, and what they can/can't expect in terms of severance benefits, etc. And get HR clarification about what to do if they come back, and what sorts of communication is/isn't appropriate after that person leaves!
Done...
Well, it went about how I expected. Wasn't pleasant, the employee cried and several of the team saw them escorted to their desk to clean it off and escorted out of the building. I spoke to each of the team one on one letting them know all they needed to know-- that the person no longer works here and it was due to policy violation. The employee has filed a grievance with employee relations because they claim they did not do what they were released for, but it doesn't look likely that the termination will be rescinded. At any rate, it's done with and out of my hands for now. Thanks everyone for your words.