Gutenberg – How to prevent block from being deleted and reordered/moved

You could use a block template.

For any given post type, you can set up a block template which adds blocks to the Editor whenever you create a new post of that type. These templates have an option called template_lock which you can set to all which means that users cannot add, move, or delete any blocks.

Of course you probably want to let people add other blocks – but you can still use a template. You would need to create your own wrapper block, one that holds your event block inside of it – and that also allows InnerBlocks. This way, your wrapper with its event will always be required, but people can still add, edit, move, and delete any other blocks on the page as needed.