Welcome to the Community, @gbowser.

 You can remove the sync data from a third-party app by manually deleting the transactions. Also, you can disconnect the app to stop the future importing of transactions.

To delete a transaction, here's how:

1. In the left navigation pane, go to Sale or Expense.
2. Look for the transaction that you wanted to delete.
3. In the footer, click More and select Delete.
4. Click Yes to confirm.

To disconnect a third-party app, here's how:

1. In the left navigation pane, go to Apps.
2. Select My Apps then look for your app.
3. Click Disconnect.

Aside from deleting the transactions, you can also void them. Click on the link to explain the difference of void and delete as well as the process on how to do it: Void or delete transactions in QuickBooks Online.

Let me know if you have additional questions by posting it here in Community. I'm always here to answer those questions for you. Have a great day.

Is there a way to remove this data in bulk?  My 3rd party ap sync'd last 5 years of data and Quickbooks didn't apply payments correctly and it is a HUGE MESS to the point where I'm wanting to just cancel Quickbooks all together. 

It's good to see you here in the Community, smagar,

I can share some information about bulk deleting transactions in Quickbooks Online.

Currently, the option to do a batch delete isn't available in QuickBooks Online. As a workaround, you can look for compatible third-party applications in QuickBooks Online.

Here's how:

1. Go to Apps.
2. Click Finds Apps.
3. Type a keyword in the Search for an app field.
4. If you have found the app you're looking for, just click Get App Now.

You can also check the reviews from the customers. This can help you decide on the third-party app that suits your needs.

In case you'll want to cancel QuickBooks Online, you can either log in to your account and cancel it yourself or contact our Customer Care Team for assistance.

1.    Click the Gear Icon.
2.    Go to Account and Settings.
3.    Select the Billing and Subscription tab.
4.    The system will ask you to answer the exit survey question and click Continue.
5.    You will receive an email confirming your QuickBooks Online subscription is canceled.

I've added this article to get more details about canceling your account in the following article: What happens to my QuickBooks Online data after I cancel?.

I hope I've covered all of your concerns today. If you still need more help, just leave a reply and we'll get right back to you.