Perhaps you can use Data Validation..

Select the range of cells and go to Data|Validation.

Select Decimal from the Allow drop down menu

and then select Less than or equal to from the Decimal drop down.

and enter 0.1 in the Maximum field.

You can further select the Error tab to enter your own error message.

Click ok to finish.

Now when somebody types a value greater than 0.1 in any of those cells, they will get a warning