This isn't really workplace sparring, but I know what you mean about friends/co-workers challenging you.
In high school, all of my friends knew that I trained. However, I was very skinny and bad at most sports, so they didn't take me seriously. One of my best friends started hanging out with us around 10th grade, and he studied an almost complete scam martial art, Oom Yung Doe. They basically brainwashed him that it was the ultimate death art and he bought it because he didn't know any better yet.
One day we were at lunch, and one of my friends said something like "I bet he (death master friend) could kick your ass because he is way bigger." Obviously this is real awkward for both of us so I said something like "Well it's pretty lame to say something like that it. It would probably be pretty even." I said this so as not to put the death master friend on the spot. To my surprise, the death master says, "Yeah, my stuff is way more powerful." I was like WTF and it was on.
My first match against him was at a party. We had no gear so we fought point style (very light contact just to show that you could hit). I basically ran circles around him tapping him in the head to show I could hit him. It convinced almost everyone else, but my death master friend still said afterwards that he won. I was like okay, he is my friend but next time he learns.
A while later we put on boxing gloves and boxed full contact. I knocked him out in ten seconds flat. It wasn't like unconscious brutal convulsions, but I knocked him on his ass and he could not get back up. After that, he stopped training as much at the scam place.
Nowadays he is one of my best friends and even trains grappling with me sometimes. He still points out some of the flaws of grappling (most of which are valid points), but recognizes its effectiveness in certain situations.
Again, it's not really workplace sparring, but I thought it fit into the thread.