Hmmm.. Depends a lot on the situation.
Are these people dangerous? Have you attempted to maybe give them a free cup of coffee and remind them about their very important meeting they need to get to? Why do they come into the shop in the first place? (are they looking to socialize, or they just randomly chose your shop?)
Making a scene is not always necessary, normally these kinds of situations can be resolved without calling the police as well. The trick is to stay calm, and reason with them. (sounds tough, but it can be done in most cases. Unless they are potentially dangerous, then you would need to call the cops...)
You can start by asking them calmly if you can help them. If they say no, ask them what they're looking for, if they came for something in particular, etc. If they say no again, ask them the reason they came into the shop. Listen attentively, and then say that it's store policy that tables, chairs etc are reserved for people consuming product. Point out that while it may not be busy (or if it is, point out how busy), and say it would be a shame if someone were to buy their coffee and not be able to enjoy it sitting down.
Of course all depends on the kind of person they are.. But I reckon that would solve 90% of your problem with harmless ones anyway...