RewriteRule ^/~account([/]?)$ / [R=301,L]
This rule will redirect anyone who will try to access /~account or /~account/ to root folder of your webpage.
"R" means redirection - the browser is sent to new address and it also change URL in URL bar. The 301 code means, that this is permanent redirection. If it is not permanent redirection, then use code 302.
Hope this will help.