I find it somewhat shameful to see both Hong Kong Chinese and expatriates using the word "mainlander" in similar vein to the word "vermin".
Yes, there are problems created by large numbers of visitors from across the border but much of this can be attributed to supply and demand, the greed of landlords or market forces depending on one's particular bias. Those who advocate that the Hong Kong government should place restrictions on the flow of Chinese tourists into the city need to take a step back and reflect on their xenophobic views.
For decades, the vast majority of our neighbours on the mainland were unable to travel outside their own country without official permission and, in some cases, their travel was restricted within China itself. In fact, the hukou system still ensures that Chinese citizens from one part of the country are unable to enjoy the same rights in another.
Today, the situation is much improved but for many citizens of the People's Republic, travel outside China remains a dream or a bureaucratic nightmare at best.
It should be remembered that aside from tourists, many visitors from the mainland are much-loved family members, friends, or boyfriends/girlfriends of Hong Kong residents. For most of these people, the restrictions placed on their right to enter and stay in Hong Kong are already draconian. Thus, meaningful relationships become unviable and people on both sides of the border are limited in the time they can spend with people they care about.
In Hong Kong we pride ourselves on carrying the banners of democracy and freedom for the whole of China; we supposedly cherish our "core values" and like to believe that Hong Kong is the conscience of China.