We have a Marketo Sandbox that we have not used in three years, and the programs and set up no longer serve our needs. I was told by Marketocares that we can't "clean it out" and that we would have to provision a brand new one. I have also been told that we could just wipe out all the leads in there and then re-sync our existing instance to the Sandbox rather than getting a brand new one? Does anyone have any insight or experience on this topic?
The best person to help with these questions would be your Customer Success Manager or your Account Executive.
You could request for a new sandbox to be commissioned -> then do an instance copy of your production instance into sandbox (goes through support) and then you can sync your leads.
PS: please note that the instance copy does not copy across data/user etc..there are certain considerations.
Hope this helps