1) Welcome to the party!
2) Newbie notes:
when you post, please post your entire configuration including (but not limited to) your installation method and vicidial version with build.
this IS a requirement for posting along with reading the stickies (at the top of each forum) and the manager's manual (available on EFLO.net, both free and paid versions)
You should also post: Asterisk version, telephony hardware (model number is helpful here), cluster information if you have one, and whether any other software is installed in the box. If your installation method is "from scratch" you must post your operating system and should also post the .iso version from which you installed your original operating system.
If this is a "Cloud" or "Virtual" server, please note the technology involved along with the version of that techology (ie: VMware Server Version 2.0.2). If it is not, merely stating the Motherboard model # and CPU would be helpful.
Similar to This:
Vicibox X.X from .iso | Vicidial X.X.X-XXX Build XXXXXX-XXXX | Asterisk X.X.X | Single Server | No Digium/Sangoma Hardware | No Extra Software After Installation | Intel DG35EC | Core2Quad Q6600
3) ssh into the server and you can terminate a call at will within asterisk
4) use the astguiclient interface which may have the ability to terminate a call (but then again, maybe not! LOL /agc/astguiclient.php). Otherwise, the ability to terminate a call does not yet seem to be something I've bumped into. haven't really looked recently, though.
5) If you cannot find it, consider submitting an issue on the VICIDIAL Issue Tracker requesting this feature and then seriously consider sponsoring its development (or at least committing some funds to it so others may join and get it built). the link is at the top of this page.