How can I prevent my unfiled TT online 2017 return info (I ended up buying the TT software to file) from being used on my 2018, as it is wrong (AGI, refund amount).

Ok ... so when you log in online you must choose the correct file from your 2017 return ....

 

If you have not paid for the online edition you are using, have not filed your tax return or registered the Free edition, then you can clear your return and start over with a lower priced edition.  Click on Tax Tools on the left side of the program screen while working on the 2017 online tax return.  Click on Clear & Start Over.

2017 Downloaded to 2018 Online

Last year's tax data file (.tax files) can be transferred to Online Deluxe and higher-tier versions.

1.    Sign in to TurboTax Online, if you haven't already done so.

2.    Select Tools on the menu bar lower left on the screen.

3.    In the pop-up window, select Transfer last year's TurboTax return from your computer.

4.    Read the warning and then click Continue (or Cancel to abort the transfer).

5.    Browse to where you saved your .tax2017 file and select it.

6.    Click Transfer Return and wait for your file to finish uploading.

7.    When you see Transfer Complete, click Continue to start working on your return.

https://ttlc.intuit.com/questions/1900883-transfer-last-year-s-return-into-turbotax-online