I need to disable or make readonly a tinymce textarea at runtime.
Use the configuration parameter readonly
tinyMCE.init({
...
theme : "advanced",
readonly : 1
});
Here is a link to a demo.
Update: What you can do to prevent users from editing content in your editor is to set the contenteditable attribute of the editors iframe body to false:
tinymce.activeEditor.getBody().setAttribute('contenteditable', false);