At the moment I have this:
tinyMCE.init({ // General options mode : \"exact\", elements : \"fkField, lkField, ukcField, khField\", theme : \"advanced\",
add in a ccs file the following code
.mceContentBody{ overflow-y:hidden!important; }
and add the path of css file in content_css attribut of tinymce
content_css : /path/to/css/file.ss