Is the variable mandatory on Task? If the variable is Mandatory on Task, it will remain "editable" until they answer the question. Once they click on "Save and Close" on the Task, on the next Task it should appear as ReadOnly.
Create a UI Policy that makes the variable ReadOnly, and it only applies on Task. Please note that this will only work if you have the variable Mandatory. If you do not have the variable Mandatory, then the variable will be ReadOnly from the start.
