I had this exact problem in my dynasty. For 4 sim weeks everyone in Sunset Valley was at the beach. However, this didn't affect my gameplay. People formed relationships, people died and were born. In fact it was kind of easy because really if I wanted to find anyone I already knew where they were. Everyone went to work like they were supposed to and then came back. Eventually I bought the property and gave them beds, bathrooms and a bar with food to better their lives. At the end of four weeks, whatever the problem was fixed itself and they all went home. Nothing else ever worked to get them to move.
So your choices are let it happen until it stops or go to a new town. Like I said, it wasn't causing story progression to stop. It was just their physical bodies that were stuck. I also started using this spot as a party spot so when I threw a party I knew the guests would arrive.
