How to make your calculator stop displaying 0.666666 when you divide 6 into 9?

Answer 1

If you divide 6 into 9, the calculator will never display 0.666...; it will display 1.5 (the correct value).

If you mean when dividing 6 BY 9, then depending upon the make and model it may, or may not, be possible. The more recent Casio calculators have an option in the setup which allow for either the fraction or the recurring decimal to be displayed:

When using LineIO, the default is to display in rounded decimal format and there is no option; however, if you have a [S↔D] button, that will rotate between fraction, recurring decimal and rounded decimal display format.
When using MathIO, you can set LineIO output (as above) or MathIO output which defaults to a fraction, but use of the [S↔D] button will rotate between recurring decimal, rounded decimal and fraction form.

Alternatively, the less recent Casio calculators can convert the rounded decimal to fraction format (and back again) by pressing the [a b/c] button.

For other brands I cannot say as I haven't got any.