the question is very general so its difficult to reply.

if as per law you have caused any damage to the reputation of any body then you can be prosecuted for the same however in your case its looks unlikely that you have done any such thing. any expression of personal feeling is normally not defamation. no body can use ur site without ur permission and if anybody uses it then you can file complaint against him/her.

If your opinion is reasonable ,then it does not amount to defamation.

usually opinion can be clearly differentiated from defamation ...