Aneuch offers several API calls to assist plugin developers, which are documented here.
Note that while it is certainly possible to call these functions in your config.pl file, they may or may not work as expected. The only guaranteed way they will work as you might expect them to is by calling them from a plugin (or registering a "late load" sub by using the RegPostInitSub API call). See Script Flow for more information.
Basically, if you want to call Unreg* on anything that was Reg*'d in InitVars, you won't be able to accomplish this in your config.pl file (as InitConfig is run before InitVars). If you wish to unregister something that is registered in InitVars, you'll have to call the unregister function from a plugin (which gets loaded after InitVars).