What's the best way to prevent abusers/bots from hitting an API?

I'll be building a small analytics app (just like Google Analytics) which will be initially used by a set of websites owned by me. Like GA, this will be a tiny snippet that will hit my server and send some data. There is no authentication or anything like that - Just an open pub-id. My question is : How do I make it difficult for bots or some random spammers from hitting my API continuously and sending fake data?