Foreach will only work if the item is in an array (and GLOBALS won't count :P) or is a traversable type of object.
You can unset as many variables as you want though. That's a simple matter of calling unset with multiple arguments:
unset($var1, $var2, $var3, $etc);
The only alternative I can think of is to provide an array of keys to lookfor, and traverse the $GLOBALS and unset. I do not suggest that route though.