You might try temporarily putting something up on the ledge to make it difficult for the birds to build a nest. I've used wadded up newspapers, stuck with double-sided sticky tape. Use a broom to knock down any nesting materials you discover. Do this once or twice a day and the birds will eventually get the message that the ledge isn't the best place to nest.
Best wishes!
You must be signed in before you can post questions or answers. Click here to join!