Lately I've been seing all sorts of strange URLs show up as errors in my Webmaster tools account, but they're URLs that don't actually exist on my site, nor are linked from the pages that Google claims they're linked from.

URL                                 Response Code   Detected
yR3kna/5RfA4+ndtn/X4zcevudMlXbqbIrnPbH9irw= 404     9/16/12
OK4iaOVdr6Ocjmz+u1kuR5Q486mhDo/e45nwjl2+y8= 404     9/9/12
pxGz/oHEA0BS8U3VFBzJcZnnIHMsFXb3/rIxMxh2ws= 404     9/16/12
Af8tbvQ0HniIpf53I8Txz1hM1/JxxrFQxgqPuErWII= 404     9/9/12
7Bk7c0LDmm4PHyTjml017EGwNNPCn/p/0xMSWWPDic= 404     9/16/12
umCwnDvTE8ybpUB19MIb+VRj5xRJncyYGGfAQ2Mxn0= 404     9/1/12
# etc...

Do you know how to make these stop? It's not at all clear to me why it would be going to these URLs in the first place.