Log in, and click on "Auto Investor Account Settings" on desktop or tap the gear icon in the app.

Go to "Auto Invest" and switch the automatic reinvesting feature to be inactive (by default, it is set to active).

Then, go to "Passive Income" and set a target cash amount. Repayments will fill your balance to this amount. If you've set the automatic reinvesting feature to "inactive", then repayments will still not be reinvesting when your target is met.