It is by design, but it is a very annoying feature.
Was this ever confirmed to be intended? It's very unfortunate if this is another one of those Get Famous "features". I can understand if it's a trait that select Sims have, but for ALL Sims you become friends with to be allowed to force themselves in is a bit excessive...
Considering you actually get a notification that says why ( you gained a celebrity star) and who, I would say that's intended like the welcome wagon but only the gurus can confirm. In city living apartments it only says neighbour, yet it can be any Sim in the world and that is intended.
The Private dwelling trait may help, have not tried it myself but have used it to delay welcome wagon while my Sim was at work. Once removed the welcome wagon turns up so may be the same thing with the celebrity level.
I gave my lot Private Dwelling as soon as I loaded my save and unfortunately it does not keep the notifications or the Sims out.
Too many bugs and annoying "features" like this one, react to disgust, and reputation cheering every few minutes to enjoy the game so I'm just going to opt out of the system.
