When nice is not so nice

So, I like to think of myself as a nice person. I'm friendly, polite, and make small talk with cashiers and waiters and people I stand next to while waiting in line. I like people, and it's natural for me to smile and chat with them. We're all human, it's nice to brighten other people's days, turn that frown upside down, yadda yadda yadda.

However! This can occasionally backfire. For example: At my work, I have to walk from one building to another, often a few times a day, because my coworkers are kind enough to help me get exercise by scheduling meetings in buildings where my desk is not located. In the course of my walk, I regularly pass by one of the security guards stationed at the entrance to the parking lot.

For a long time, I'd pass by in silence or with a smile. Then we progressed to "how's it going" and platitudes about the weather. A week or two ago, the guard, D, introduced himself, saying he figured he should since we see each other all the time. Now we have moved on to three-minute conversations every other time I pass by. I've had small talk about parking tickets, weekend plans, and tans. If I don't show up in the morning, I get comments about 'where were you this morning, I missed you' in the afternoon.

Someone get me out of here.

I may think of myself as a people person, but even I don't want to be chatty and friendly all the time. Can't I ever walk by in a black mood and not want to engage in some small talk? Can't I ever want to walk straight to a meeting? I think YES.

So...what do you think? How do I fix this, remedy the acquaintance I seem to have unintentionally formed, so that I can walk from place to place undisturbed? I don't want to have to plan a route that takes me 10 minutes out of my way around the back of the building. I just want to be scowly and silent when I feel like it. Help me be hostile, pretty please?