It is never a good idea to turn business away: it just creates tremendous bad reputation in the marketplace. What you want to do is to manage your growth strategically by managing the pipeline in a way that you can handle it yourself or pass it on to a reliable (as good as you) partner. That way when you do need additional business you can go back to these customers and ask for new business.

It is also important not to do a shoddy job just because you have too much work. That is even worse.

The third dimension is to think hard when you should quit your day-job and become an entrepreneur. If business is good it is not a bad idea to ride the wave, hire staff to do additional work, and create wealth.

Good luck.