I had this same problem and appreciated a way to fix it, but it didn't solve the problem on a global scale. So, after doing the clear contents and all, I tested if it would work going forward by adding a new column. It did seem to prevent new columns from having this problem, which is lovely. However, I didn't want to have this happen ever again, and I use calculated columns a lot. When testing, a little box appeared floating just below the column header. When clicked, it had several options to control the Autocorrect behavior.

If you open AutoCorrect options, on the AutoFormat as You Type tab, the last item (in 365) is "Fill formulas in tables to create calculated columns." Unchecking this should prevent the creation of columns with unintended formulas. You can also access the options by clicking the following steps: File tab, Options at the bottom of the list, Proofing in the popout menu, the AutoCorrect options under the AutoCorrect Options header.

I'm adding this note simply because I would never have guessed AutoCorrect was at the bottom of this.