You can do this without deleting/importing those you mailed to in the last 6 months.
You need to:
Find all subscriber records that have existed for more than 6 months
Add these to a list in your admin account
Delete that list and when you do so elect to also delete the subscriber records
to obtain these records you'd want to make use of a query:
Here is your query
a.SubscriberKey as 'SubscriberKey',
a.EmailAddress as 'EmailAddress'
FROM _Subscribers a
WHERE a.DateJoined < DATEADD(day,-180,GETDATE())
Create a data extension and direct the query to overwrite to this.
Export the data extension and import it into a list
then you just need to delete the list and when doing so also elect to delete the subscriber records
You need to do this in your admin I.E highest level account
Furthermore it's possible to automate this whole process bar the actual deletion of the list if desired.
This can be done by adding a data extract, file transfer and import file activity to the automation with your initial query.
Allowing you to automate the extract of the data extension and import into your chosen list.
It's also worth noting that deleting subscribers can take time backend and you will need to delete lists of no more than 500k in size.
A easily way around this is to split your list into several groups and delete these individually.