我怎么能阻止用户在ASP.Net的文本框中输入回链接

I have website which is luckily on top of Google but there are spam messages with backlinks of different websites from spammers too so what i thought a solution to prevent visitors from putting back links in Multiline Text Area ? I have tried with required field validation and some other ways but unlucky yet. I am bit familiar with Javascript but after searching i did not find any javascript to prevent backlinks in text area. If any one can help me a bit or just give me an idea ?I have website which is luckily on top of Googl