If you are able to move items from a Resolved to Closed state without a signature, it is likely that the workflow in Jira does not have the condition “Hide From User Condition” enabled on the transition from Resolved to Closed.

To add this condition, select “Add condition” seen in the above screenshot, select “Hide From User Condition” from the list of options, and press “Add” at the bottom of the screen. Once the change is made, make sure to select “Publish Draft,” so any changes you have made are implemented.