Dissecting server error log - text+captcha+decoded

[DF031]

Good evening all,

In the server logs of our forum we get hundreds of errors like these every hour

I edited the green text and. I especially worry about the red stuff

- - - - - - - - - - - - -
[Thu Sep 26 20:21:12 2013] [error] [client 192.187.125.187] File does not exist: /home/XXXXX/domains/XXXXXX.net/public_html/index.php+++++++++++++++++++++++++++++++Result:+te xt+captcha+decoded;+chosen+nickname+"acensebak";+registered+(registering+only+mode+is+ON);+Result:+chosen+nickname+"Woftod odrurse";+registered+(registering+only+mode+is+ON);, referer: http://www.XXXXX.net/index.php+++++++++++++++++++++++++++++++Result:+te xt+captcha+decoded;+chosen+nickname+%22acensebak%22;+registered+%28registering+only+mode+is+ON%29;+Result:+chosen+nickname+%22Wo ftododrurse%22;+registered+%28registering+only+mode+is+ON%29;
- - - - - - - - - - - - - -

The names Woftododrurse and acensebak are not unique, they are used over and over again.

What is this ? Should I worry ? Should I stop it ? How to stop it ?

Does anyone have any additional about ths ?