At the moment, this feature is not available.
For now, you can delete your entire project—this deletes all the snippets in it.
If you only want to delete some snippets from a project, you'll need to do it one by one.
We've created a post based on this request, here: https://1se.canny.io/admin/board/feature-request/p/ability-to-bulk-delete-snippets-ios.
Please take a look at it and upvote it if you think it covers your needs—feel free to include any comments or images that might help support your case.