A couple of ideas come to mind. Put a rubber snake in the pot, that should frighten the bird away. My Grandmother always had one hanging on her front porch to keep the birds from nesting in the porch columns. Creepy, if you don't like snakes, but effective.
Drape some netting over the pot to restrict access to the nest, and the bird will move elsewhere.
Of course you could also tear out the nest, but if it already has eggs in it, I wouldn't have the heart to do anything except move the pot a little further from the entrance of the house.
BBH

Actually, I got a better look at mother bird from across the yard. She seems to be a dull-colored robin. While she was away from the nest, I got up on a ladder to look and see what I was dealing with, and she has already hatched some baby birds. I have been regularly watering the nest out of ignorance until yesterday, and so far the babies seem to be breathing but not chirping. Also so far, I have not accidentally drowned them. I have a friend that has an extra shepherd's hook, and I may move the nest to under a tree next to my garden in a protected area, so that the birds can develop without all the comings and goings of my house. Also, if they fall out, they will land on soft dirt and not the hard cement of my porch. This just feels so unnatural and a little strange to me...

You can't let it be until they fledge? The mother built her nest there for a reason. In a couple weeks they'll be gone and you can have your plant back.

Here is a link that might be useful: robins nesting habits

Every year, there is usually a Carolina Wren or a Sparrow that wants to nest in my containers. I let the Wren take over one planter on my patio underneath a heuchera that I could water from the bottom without bothering the nesting birds. Currently, a Purple Finch pair is nesting in a potted arborvitae.

However, on my front porch my containers are right under a ceiling fan -- not safe for flying birds! I used leftover green plastic forks (kid safe) and planted them around in the pot with the tines up. The tines are very dull, but this keeps the birds from nesting -- much safer than an encounter with a ceiling fan. The forks aren't even noticeable among the flowers and foliage.

Cameron

What a great idea, Cameron. I had the same problem yesterday but I caught it before the nest was built. Wrens. They had just atarted. I took the pot down, took the nest out, and soaked the plants with bloom booster (something I was going to do anyway when I saw Mr. Wren flying in and out of the pot). They had already finished one brood in a cardboard box in the garage ... laid, hatched and flown the coop. Sorry. One free apartment per year is my limit.

The plastic forks are the perfect solution. I was also going to try sweet gum balls (I have an overabundance), but they would probably try to make them part of the nest.

Thanks for all your good feedback. I think that moving the nest may be more disturbing to the mother and babies than leaving it, and the new location not as sheltered as I would have liked. I have the shepherd's hook, but it is not as tall as I had hoped, so the mother and chicks would be too exposed in my mind. Since things have progressed as far as they have, I think I am kind of stuck with the situation this time. I will try the plastic forks next time, but in all my years of gardening in my yard, this has never cropped up before--so totally unexpected! The plants are only geraniums and nemesia, and if the bird had not already laid eggs and/or hatched them, they would have been worth saving to me. It just looks like such a precarious spot to raise little baby birds.