Data validation in Excel can't directly prevent users from changing formulas within cells. It's designed to restrict the type of data entered, not modify existing content.

However, there are a couple of workarounds you can consider:

1. Cell Protection:

- Select the cells containing the formulas you want to protect.
- Right-click and choose "Format Cells."
- Go to the "Protection" tab.
- Check the box next to "Locked" and click "OK."
- With the worksheet protected (go to "Review" tab > "Protect Sheet"), users won't be able to edit the cell content, including formulas.

Important Note: Remember to unprotect the sheet when you need to modify the formulas yourself.

2. Hide Formulas:

- Right-click and choose "Format Cells."
- Go to the "Number" tab.
- Select a custom format code that doesn't display the formula value (e.g., ";;").
- Click "OK."

While this doesn't prevent formula changes, it hides the actual formula, making it less obvious for users to modify it accidentally.

Additional Considerations:

- These methods restrict user interaction but don't prevent someone with access to the sheet from modifying the formulas directly.
- Consider using a separate sheet for your formulas and referencing them in the main data area. This can offer some level of separation and protection.
- VBA macros can be used for more advanced formula protection, but this requires programming knowledge.

For robust formula protection, consider using password protection features or collaborating in a controlled environment.