The anti spam plugin has problems that it bans total ISP such as cable and mobile phone users. One must take care when banning IP its never a good policy to use third party data.
I have had so much trouble caused by this plugin and have adopted my own methods to keep my sites spam free. It very simple just create a green room default role and turn off allowing users to publish any thing on your site. Then vet each user its normally very easy to spot a spammer such as the avatar that mimics the default avatar but has a border and of course how the profile questions are answered. The genuine users can then pass vetting and the role changed to member.
The only problem at this moment is the fact that you can not un tick the default user. So you have to open two windows one to drag the default role down and the other to change the new users role. Then drag the default user back to the top of the list.
The princible is the same as newsvine.com has been using for 5 years or so to keep their network clean of spam.
You can delete your spammers before they plaster your site and they get fed up eventually and leave you alone. I have three oxwall sites spam free, with little effort to keep them so.
Nirmalya, if you get too many of them your might run out of space on your server and this could slow down your site. But there are many other reasons that slow the site down. All depends on your server settings and how Oxwall is set to run there.