The default settings on Ventrilo don't allow you to perform all the admin functions when using just the rcon password.
While logged in with rcon, right-click anywhere within Ventrilo and go to Server Admin -> User Editor. Go ahead and create a username for yourself and assign a password. Then, go up to the top and choose the "Generic Admin" profile from the drop-down box and click on the [Load] button. Doing this will load some default privileges for you.
At this point, go over to the Admin tab and make sure that you check every single checkbox on that page. Then, just go ahead and click the [Add] button. You should then notice your username under the list of "Current Users".
After this, just go ahead and log off of the server, and then log back on using the Username and Password you just set up via the User Editor. Now you should have the option to kick and ban users. To do this, just right-click on that user's name and go to Server Admin -> Kick or Ban, depending on what you'd like to do.