Learn Finnish: Jos et käytä palvelua, tili passivoituu. - If you do not use the service, the account becomes passive.

The suffix -oitua attaches to the loanword root for 'passive' to create an automative verb. This indicates a process that happens independently to the subject. Instead of a person explicitly deactivating the account, the account simply 'becomes passive' on its own.

Since passivoituu means the account becomes passive on its own, what verb would I use if someone actively deactivates it?

You would use the transitive equivalent passivoida (to make passive / to deactivate). The -oida suffix implies an external agent is doing the action to a direct object, while -oitua means the subject is undergoing the process itself.

Is palvelua in the partitive case because of the verb käyttää or because the sentence is negative?

Both! The verb käyttää (to use) generally takes a partitive object when referring to an ongoing action like using a service. However, because the verb is negative (et käytä), the object must be in the partitive case anyway, which is an unbreakable rule in Finnish.

Why is there a double 'u' at the end of passivoituu if the dictionary form is passivoitua?