Great question! We don't currently have the option to delete values based on conditions like this but may be part of an update down the road. For now, you can still have a separate view in your table where you filter and surface them, so then you can go into once a week or so and delete them
If you delete the record, yep, the enrichment gets lost. If you don't want to enrich those that don't exist in your CRM, you can still use their Lookup feature to determine if you have them or not, then that can be part of subsequent conditional formulas that only run if they do exist.
Maybe that could be a workaround, adding those separately to another Clay DNC table... Here's an example of how to setup a lookup for DNC - https://www.loom.com/share/80d613c10d5248f6bfc662cc95c8023d
Based on matches, you can set up conditional formulas so existing records don't get enriched or contacted.