Try:
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewiteRule ^([^/]+)$ ./poll.php?code=$1 [L,QSA]
And if you need to redirect the browser when requests are made directly to poll.php:
RewriteCond %{THE_REQUEST} \ /+poll\.php\?code=([^&\ ]+)
RewriteRule ^ /%1? [L,R]