Restrict web addresses in text box

Hi,
I am getting quite a bit of spam recently and have read the FAQ but I would really like to stop anyone from entering web addresses in the text box.
I am looking for the custom code that would stop this from happening.
Here's an eample of what we are receiving.
Can anyone help me please?
thanks
James

Hi,
It would be great if someone can help me with this please.
Thanks
James