odd 404 errors

the hosting company I use sends me e-mail reports whenever a 404 error occurs. the problem is that the “pages” the supposed user is trying to find are items for which no links exist on my site (or anywhere else).

here’s an example:
https://fitcommunications.net/2005/02/15/spring-in-my-step/this.options%5Bthis.selectedIndex%5D.value

this is the actual post as it exists on my site:
https://fitcommunications.net/2005/02/15/spring-in-my-step/

the second oddity is that the e-mail report the host sends indicates where the “user” came from. In several cases, the “user” has come from https://fillbest.com

a google search reveals that fillbest is blacklisted for creating referrer spam.

can someone enlightened help me figure out what’s going on and how to fix it?

thanks,
Mary