The readonly attribute only prevents the editing of an html INPUT, not the copying of text anywhere. So it’s going to be somewhere else.
Or did you mean giving someone the ability to change a text input field, at which point, then yes, removing that attribute will allow the input field to be altered.
Note that is is pretty much impossible to do what your you are trying to do. While you can use fancy JavaScript to block the user, all somebody has to do is…
A-View the source code
B-Inspect the site then copy the text from the source code
C-Turn off JavaScript
D-Take an image and run it though text recognition software