Adnan Gujjar on "how to secure plugin area to be exploited?"

hi,
my site http://www.filmylinks4u.net/ has been hacked many time :(
after investigation i found that during each hack plugin files are changed and virus is injected into it, and some time there are new folders created like freecredit/online credit card etc...
my question is that how to secure plugin area ???
any suggestion/help will be appreciated.