Papillons are generally very obedient and docile dogs. They love pleasing their owners and rarely persist in a conduct that they notice that their owner disapproves of. However, they can sometimes bark incessantly and without obvious, or, at least, good cause.
If you live in an apartment this might present quite a problem. In order to stop your dog form doing this you need to try to understand his reasons for behaving in such a manner. One of the possible causes for such behavior is that the dog is bored or frustrated and that he wants to attract your attention. This might be a part of a more serious problem, known as separation anxiety, which should be a topic of a much longer discussion and cannot be addressed properly here, or it can just be temporary boredom. If you do give in and play with your Papillon it will probably silence him for the moment, but it will also show him that his methods for attracting your attention are effective. If you intend to stop him from trying to attract your attention in this way, you should completely ignore him and show him that he is getting nowhere with his behavior.
At other times the reasons for barking might not be that obvious. He might have heard a noise that he absolutely must alert you of, or he has seen someone out in the street and is trying to chase that person off. Whatever might be the reason, you will want to stop him from acting in such a way. One of the possible ways to do this is by filling a can with pennies or other small metal objects. You should shake this incredibly annoying rattle every time he barks without proper cause. It should silence him immediately. Once he has stopped barking reward him. In time he will learn the correlation between his silence and the reward.