Hi Elsa,
Johan is right, there is currently no great way to cancel an in-progress import without potentially causing data corruption (i.e. incomplete rows in your database that can lead to unexpected problems). However, you also asked about pre-validation.
AtoM does have a CSV validation service that is supported both as a separate test, and as a setting you can apply to run automatically before every CSV import. You can also run either from the command-line or via the user interface. See:
For adding it to CSV import attempts in the UI automatically, see this administrative setting:
NOTE: It is a bit hard to use the Strict mode, because warnings are generated for many valid events - for example, if your import will be creating a new repository record or a new authority record, that triggers a warning (just to ensure this is intentional). To avoid such warnings blocking your import, using Permissive mode allows for warnings but will cancel the import if any errors are detected. Additionally, if the import does proceed, warnings are still visible in the job details log.
Cheers,