There is no such option because votings are meant not only for Forza Horizon 5 but also Forza Motorsport and other future games 
If you want to cancel your vote you can do it by clicking the blue “Voted” button.
As for cars that are available in FH5, you can find the complete list here:
(it’s missing only cars added with Rally Adventure expansion and Series 19 update at the moment, but I’m sure Max will update it soon )
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