Simply remove Skype from your Login Items in System Preferences. You'll find that under Users & Groups. Select your admin account, then choose Login Items. Scroll down the list until you find Skype, then select it. Finally, click on the subtraction symbol beneath the scrollable list. Skype will not load when you next start your computer. Note that simply unticking Skype will NOT prevent its loading at startup, but will only hide it after launching.
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Entia non sunt multiplicanda praeter necessitatem.
