I've had some users trying to access a site that is registered as subdomain.example.com with www.subdomain.example.com.
is there some sort of .htaccess rule I can add to redirect people that arrive using www.subdomain.example.com to subdomain.example.com?
Also, do I have to change DNS stuff?
Sure, use a directive like:
<VirtualHost *:80> 
    ServerName www.subdomain.example.com 
    Redirect permanent / http://subdomain.example.com/ 
</VirtualHost> 
Apache automatically preserves anything after the / when using the Redirect directive, which is a common misconception about why this method won't work (when in fact it does).
Also, yes you will need to change DNS records, because www.subdomain.example.com is a distinct hostname that needs its own A (or CNAME) record to point the browser to an appropriate server in the first place.
RewriteCond %{HTTP_HOST} ^www.subdomain.domain.com
RewriteRule (.*) http://subdomain.domain.com/$1 [R=301,L]
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