Hi,
I suggest you refer to this support article:
https://support.microsoft.com/en-us/office/send-to-kindle-a53d880d-9952-4bf1-abc5-6bce8db5a273
As the article said:
To disable user access to Send to Kindle, you can go the cloud policy link, locate the [Turn off Send to Kindle] policy and set it to [Enabled]. The policy will automatically be set in place and users will not have access to this feature. Alternatively, you can set your optional connected experiences policy to off thus disabling all optional connected experiences for your users.
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