There is currently no way to delete multiple products from the DB.
You can however delete multiple product Variants from Catalog > Products > Bulk Edit Product Variants
It would be possible to build a bulk delete products function.
I would build it as a button on Catalog > Products > Manage Products
In fact you could probably use the import export functions to do bulk deletes at least review how they work to give yourself a handle on how to build a bulk delete function.
Finally you could bulk delete from SQL Server Managment Studio if you have access to that.
Note Products records are never deleted from Nop - a flag is set Deleted = true on the record. I figure this is to do with not creating problems with foreign keys that use the ProductId.