Site being constantly searched by other symbio sites

I noticed this afternoon that my site is being constantly queried (every 10 -15 seconds) by other Symbio sites – currently a lot from DPstockphotos.com with strings like this:
GET /search-images/banner/?symbiostock_network_search=1&symbiostock_network_info=1&paged=1&cr=SiteExplorer HTTP/1.1

My performance is way down in terms of page load time, and I wonder if this is the cause. Did I do something to kick this off, and can I stop it?

Steve