Is there a way to "unsummon" a summoned creature? It appears there isn't, at least from my fruitless Google searching. Would be nice to not have to wait for the summon time to run out.
I recently picked up the game after I played the first one a few years ago. You can "unsummon" a summoned creature by using the Summon Teleporter spell. In a few seconds time you're back and forth, so that's better than waiting till the timer runs out. Just be sure that you don't have the old creature on autocast :)
I know I'm about 5 years too late, but I know the answer to this!
You can desummon a summoned creature by canceling the associated buff. There are two methods to cancel a buff.
Method 1: On top top left, on the party member UI, there will be a portrait surrounded by small icons for every effect currently on the player. Friendly buffs can be canceled by right-clicking the buff icon for the buff you wish to cancel. http://screencloud.net/v/r5x5
Method 2: In the inventory screen, there is a button labeled [view]. If you click this button, you will be shown a rendering of your character. In addition, this small window that pops up will have graphical icons for every buff currently effecting your character. As in method 1, you can right-click these buffs to cancel them. http://screencloud.net/v/l6E1