| IIRC (it's been a while since I've tried that particular spell college) Summon Undead counts as a persistent spell, which means you can end it at any time by clicking on its active spell icon, found at the top center of the main screen. (Look for the row of hollow boxes, which tells you how many such spells you can maintain at one time.)
Alternately, all spells with a maintenance cost automatically end whenever you sleep -- or of course, if you faint from exhaustion.
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Speak softly, and carry a big stick. Or better yet: a big, sharp blade.  | 
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