Thanks everyone for your interest in improving English language strings.
As Koen says, errors should be corrected. However, with 'cannot' and 'can not', there are some situations where 'can not' is not actually wrong, it's just that we might prefer 'cannot' from a style perspective.
If there is a particular situation where the use of 'can not' may be misunderstood though, such as 'You can not enrol yourself in this course.' as you explain John, then the string can be changed. Thus, I have changed it in the English fixes (en_fix) language pack.