Hi there,
Multicurrency is an awesome way to broaden your business horizons. Businesses all over Canada are looking for ways to open their doors to foreign customers, and QuickBooks Online is here to support that. I'll explain why you're unable to turn off the multicurrency feature.
When you first activate multicurrency, you're given a warning that once turned on, this setting cannot be turned off.

For the integrity of your bookkeeping, this setting must remain on. The system will need to keep the feature activated, as there may be foreign exchange information stored in the system that you'll need to account for.
I know you may not have the need for multicurrency anymore, however the setting will stay active.
I hope this clears things up for you. If you have any other questions, please reach out to our tech support team.
Cheers.