How can one stop their shares from being lent?
Instruct your broker.
And if my contact with broker is via app, Freetrade...?
If there's no tick box or option in the app, email them with your account details and explicitly instruct them to not lend out your shares.
Have a read through your terms and conditions, it will be mentioned somewhere. Although bear in mind that this is part of the reason the app is free without trading fees.
@Adhiero started
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Instruct your broker.