I found an offensive site to Islam. Can we do something to explain the truth?
You have discovered pages at a particular site, for which you have asked a proper rejoinder, that indeed show hostility towards the religion of Islam, ridicule the personality of our Prophet (peace be upon him) and carry false accusations and distortions. However, the matter is as said by Allaah (azza wa jall) (interpretation of the meaning):
"They want to blow out, with their mouths, the light that Allah has sent, but Allah has decided to bring this light to a completion even though the disbelievers do not like it" (Al-Tawbah 9:32)
I do not see any point in announcing the URL of their page in any other Islamic page which Muslims visit frequently. We should not propagate its existence or invite people to it even by indirect means. What I see as important is that some capable persons should send the proper translation of meanings of the following verse of Qur'an and other such verses to them:
"Those who disbelieve spend their wealth in distracting people from the right path, will continue to spend until it becomes a source of regret for them and then they will be overcome. And the disbelievers will be resurrected towards Hellfire so that Allaah may distinguish the evil from the righteous"
It should be enough to send them this and similar verses of Quran in rejoinder and to raise their anger. Doing so will be an act of worship and nearness to Allaah as Allaah (azza wa jall) has informed about the true believers (interpretation of the meaning):
"They suffer neither thirst nor fatigue, nor hunger in the Cause of Allaah, nor they take any step to raise the anger of disbelievers nor inflict any injury upon an enemy but is written to their credit as a deed of righteousness. Surely, Allaah wastes not the reward of the righteous"(At-Tawbah, 9:120)
I ask Allaah to reward you for your religious spirit and protect and support you.