RewriteEngine On
RewriteRule ^!(.+)[/]?$ user.php?user=$1 [L]
should do it for you. The slash at the end of the URL is optional with [/]?.
EDIT:
, . CSS, JS, .. ; , , URL-, .
URL- ( ) URL- :
RewriteEngine On
RewriteRule ^!([^/]*)/$ /!$1 [R=301,L]
RewriteRule ^!(.+)$ user.php?user=$1 [L]
... untested.