b2evolution used to publically display referral stats. Spammers would send fake referrals to improve their search engine ranking by getting free links from your site. b2evolution no longer displays those stats publically, so the spammer's can't get any benefit from it. You can just delete those stats or ignore them.

They don't send any referrals as explained above. They just search my blogs for the same terms all the times without linking to their own sites

Oh, I see. Sorry I misunderstood. I can't imagine what they would gain from that. Maybe they're looking for a relevant post to spam.