Simply remove the
$ from the regex, and anything after the ID number will be ignored, and the URL will be rewritten correctly.
RewriteRule ^(.*?)\/? index.php?u=$1 [L]
# the following will work the same (as far as I can tell), and
# it's a lot more obvious at first glance what it does, which is
# match everything until the first slash
RewriteRule ^([^/]+) ...
When I do something like this, I like to verify the URL in code, and 301 redirect if the "Anything-I-want-here" doesn't match the data.