How can I disable download in shared albums in icloud?

It is not possible with shared albums. Shared albums are for sharing photos and videos you want to give away. Try to find third-party a software to create a web gallery, where individual photos and videos are protected from downloading. But even then people will be able to create screen captures of the videos or photos.

Alternately add a big watermark across the photo or video you are sharing, so the downloaded version will be useless.

See these articles https://support.apple.com/en-in/guide/iphone/iph3d2676c9/ios

https://support.apple.com/en-in/guide/icloud/mmc0cd7e99/icloud

it depends upon the invitee to accept the invitation or unsubscribe , once the photos are shared through albums they will be synced in photo application , they can export photos to download folder in the finder , delete them its all up to their discretion , you have no control over them what they do with photos see the article https://support.apple.com/en-in/guide/photos/pht6e157c5f/mac