If you want to keep your contacts in a flow until they've either completed it or expired from it, you can configure it to ignore keyword triggers until your contacts' have left it.
To tell a flow to ignore keyword triggers:
1. Navigate to the "flows" tab and click the flow you'd like to modify.
2. click on the gear icon in the top right corner of the flow editor, then click the "Edit" option.
3. Click the "Ignore keyword triggers while in this flow" box.
4. Click the "Save Changes" box to complete your flow edit.