What can I do to prevent the assignment of a short option?

The only action one can take to prevent being assigned on a short option position is to buy back in the option prior to the close of trade on its last trading day (for equity options this is usually the Friday preceding the expiration date although there may also be weekly expiring options for certain classes). When you sell an option, you provided the purchaser with the right to exercise which they generally will do if the option is in-the-money at expiration.

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Options assignment prior to expiration
What happens to my options spread at expiration?
What happens at expiration to my short option?
Can you exercise the out-of-the-money long leg of my spread position only if my in-the-money short leg is assigned?