This solution is similar to one that I was thinking of. I don't have code but the algorithm is like this:

• Pick a point where there's a complaint.
• Draw a circle of some arbitrary radius around it (a little experimentation will find the optimum radius, I'm sure).
• If another point falls within the radius, increase the area of the circle by a fixed amount and recenter it so it lies at the midpoint between the two points.
• If another point falls inside the circle, increase the area of the circle again and recenter it so the center lies as close to the center of the cluster as possible.
• Keep increasing the area and recentering as long as more points fall inside the circle.

Since you're increasing area instead of radius, the circle will stretch less and less each time until it stops growing. If the circle stops growing, record the center of the cluster and all the complaints inside it. Once you have a cluster, move on to a point that isn't part of a cluster yet and go again. If some point lies in two different clusters, make it part of the closer cluster.