Perhaps @heather or @brasten or @tom can confirm, @aronos, but I think @Blake is correct that deleting the account from the Tiller Money Console will remove the plumbing for gathering new data but will preserve the data that is already in the feeds-service back end. ← EDIT: THIS IS INCORRECT (see below)
As I type this though, I’m wondering how you’d access that if you wanted to start a new spreadsheet and pull in the historic data if there was no reference to the account on the Console UX. 
(Of course you could pull over that history manually or via the Migration Helper.)
Perhaps the best approach is to unlink the obsolete account from the spreadsheet but leave the connection intact in the Console. Then, if you started a new spreadsheet, you’d be able to link the account to the spreadsheet and pull down those old transactions.