Avoid suspected files

Hi,

My site being hacked few days ago. I have removed all of the suspected files following the suggest of hosting admin.

But my firewall plugin still keep notice me about the strange file forum.php inside the directories below everyday. Although the plugin bloked it but it automatically re-created somehow.

http://www.xxx.com/vn/xxx/data/2010/10/forum.php?amp;aid=37639&atid=10302&key=d90c995e68a702714d79b76949a3f00c&size=100x100&mod=image

http://www.xxx.com/vn/xxx/data/2012/11/forum.php?amp;aid=37676&atid=10316&key=f665adddaee5bf4a4b21c762f807646b&size=100x100&mod=image

http://www.xxx.com/vn/xxx/data/2014/01/forum.php?amp;aid=37597&atid=10286&key=139bbf5aaea964a502363debb9f19d5c&size=100x100&mod=image

I don't know where is the files come from? That directories are just contains images. How can i stop this attacking? How can i avoid the auto-create files? (p/s: i'm using very complicated FTP and admin password).

Thanks,
Tuan.