"they all have gmail account, which i really cant ban"
But, you can report to Google, if you want.
My biggest problem lately hasn't been splogging signups, it's been them swiping my rss feed and republishing it for their own google ad exposure. Which, if anyone is going to make a penny off my content, its me. Not to mention violating copyright laws, etc.
My answer to this initially was to just add on my copyright statement to feed items, after the post.
While that did deter a few people, it didn't deter them all.
So, I switched it up. Created 10 different versions of my copyright notice, moving and altering slightly just to throw off a filter, plus I intermingle an "authentication code" that changes with each build, as well as it's location.
Then, I stuck this before the rss post content, AND description.
Not much effect on legitimate readers, but it sure does kill the content lifters out there.