Correct spelling, explanation: the form seized is the only correct spelling: just look at its etymology to be sure. It was formed by adding the -ed suffix to the root (seize). We can’t change the order of the vowels in this word. Siezed is just a misspelling.

Definition of seized:
verb, took advantage of an opportunity
Anthony seized a chance to change his job when the rival company offered him far better conditions.
Spain’s police have recently seized 52 tonnes of marijuana and detained 20 people.

Incorrect spelling, explanation: the error may occur due to the fact that in the correct version of this word (seized) the first two vowels are located next to each other. It sometimes happens that some English users swap these vowels. You have to be careful about it and always check how the word-formation basis is built. Seized is the correct form, as opposed to siezed.