Say "hmmm, interesting" while nodding your head and simultaneously nodding your 'other head' so the stream of urine moves up and down. That should discourage 90% of them.

I turn around to who's talking to me.
And as a result piss on his head.
Because I'm tall.

*goes to reread entire "pee in the sink" Askme thread and its MetaTalk*

I say "Just a second, honey," tap my ear, turn to the guy -- this next part is key -- I look him in the eyes, then look down at his penis, then back to his eyes, say "Excuse me, I'm on the phone," then tap my ear and say "Go ahead honey. No, of course I'm at my desk. How else can I write it down?"

If it's my boss, after making eye-penis-eye contact, I tap my imaginary bluetooth receiver and say "Sorry honey, my boss is calling on the other line." Then I turn to my boss, look directly at his penis, and say "What was that all about?"

It always weirds me out that men's room design makes this such a problem for guys. With women's rooms, sometimes there's chat if you've sort of had a reason to bond, or if someone's run out of TP in their stall, and sometimes there's not, and either way it's never terribly anxiety producing. It seems that having the exposed urinals necessarily demands this uncomfortable protocol, and that the stress it causes is really undue. I would think it would be different if the default were stalls, but maybe not.

In short, some of this is probably a design issue and some is a socialized gender issue, but I'm not sure where the slider lands between the two ends of that range.

I respond, in both cases, in a professional manner. What else would you do?