I sense a bit of confusion here. The Plesk Server Administrator (PSA) is merely an interface to the underlying system, which includes your web server. In other words, on a dedicated box, Plesk is used to configure your operating system (such as, Redhat Linux), the web server (hopefully, Apache), and such things as SSL, SSI, PHP, CGI, MySQL, Perl, MS FrontPage, etc.
However, Plesk doesn't determine the behavior of htaccess files- the underlying system and server limit what is possible and their configuration files specify what's implemented of those possibilities and how.
My suggestion would be to install Webmin, from
http://www.webmin.com/, which will give you an excellent alternative administration interface. Or at least read the documentation at
http://www.swelltech.com/support/documentation.html, which gives a very good oversight on web site administration and configuration files.
As far as your specific problem (
"htaccess will not ban ips if I use the Limit/get post/deny from commands"), check to see if your config files have a "Directory" directive that has an "AllowOverride Limit" (with other options, such as "AuthConfig") right after the "Options None" line.