Yes I know personally of one case (actually it was a mother coming here from Korea to look after her grand-daughter while her daughter worked). After, I think, 4 renewals of her 3 month visitor visa she was told that the next one was for two weeks and that would be her last for a while. I believe she had to stay out of HK for 9-12 months before they would grant her another visitor visa.
And there have been a number of reports of similar situations both here and on other similar forums.