Selected Answer
Select the cell to limit and hit Alt + D + L and then under the settings tab go to Allow and select Custom and input this formula:
=A1<A2
Now you can't enter a value in cell A1 that is higher than cell A2.
If you want to highlight the cell use conditional formatting. Select the cell > go to Home tab > Conditional Formatting > Highlight Cells Rules > Greater Than > for the input on the left of the new window, delete its contents and, while the cursor is still blinking within the input area, select the cell that this cell should not be higher than and click OK.