Usually the wasps are just looking for a place to nest, EP. Since they build their nests in crevices, it is only natural for them to discover one that, when followed, leads to the inside of your flat. Often these potential entrances are difficult to detect. Worse, they are sometimes gaps that you wouldn't want to chink because to do so would interfere with the opening and closing of a window, for instance. I guess what I am trying to say is that you may need to accept that you are unlikely to be able to find and block all the potential entries.

Wasps dislike disruptions in air currents. You can use this to your advantage when trying to get rid of one. Just open a window or outside door and using a magazine or newspaper, gently fan the wasp toward the opening. Gently is the key word here. What you want to do is to give the creature an urge to be elsewhere. "Hey, little one. Wouldn't you rather be outside?" (Yes, I talk to wasps. Wanna make something of it?) If you are too vigorous, the wasp will become agitated and will fly in something other than the desired direction. Be patient. Just remember that wasps tend to fly in circles - small ones when they are just moving about normally; but large ones if they feel agitated or trapped.