Thanks for reaching out to us. Allow me to address your concern about inactivate class in QuickBooks Online (QBO), so let me get this straightened out.
For the meantime, you can inactivate Classes one at a time since inactivating it by batch is currently unavailable in QBO. I can see how this future would be helpful for you in order to delete Classes conveniently, I'd recommend sending feedback directly to our product developers for consideration in the future updates.
Here's how you can send feedback:
Click the Gear Icon.
Click Next and choose Feedback under Category option.
Let me walk you through on how to delete classes:
Select the Gear icon.
From Lists, select All Lists.
From the drop-down under the Action column, choose Delete.
When asked Are you sure you want to delete this?, select Yes.