Untickable checkboxes

I need to use untickable checkboxes. They should start as checked remain checked when trying to untick them. How is this possible?

Try

<input type="checkbox" name="cb" checked="checked" onclick="return(false);" />

david_kw

That works as expexted. Thank you very much, David.

Bear in mind that this isn't going to work for non-js users.
If it's for a legal consent/agreement checkbox, then you might consider other ways to 'enforce' the condition or validate the state of the checkbox.

Just a heads up.