Yosepgr,
First it's important to not spell important as "impotent", especially after Irving's reference to viagra.
Write something like this:
I noticed your website at {put web address here} was linking to mine several hundred times. I visited your site to see why you liked my site so much and noticed that it's because your website appears to have been hacked.
You should consider cleaning up all of the links the hackers put in to your website so that you do not get a penalty from Google. You should also consider taking some safety precautions to prevent this from happening in the future. Some things you can do is check your site to see where SQL Injections may occur. You might also put in Captcha validations where your users are able to input information/register for your site.
I hope all goes well with your cleanup.