SLA Violation Questions

We currently base our SLA's on Urgency and Impact. The problems we are having are when we have resolved a call - within the SLA - and the requestor either responds to the call with a "Thank You" instead of accepting the resolution or weeks later re-opening the ticket which is causing an SLA Violation. Here is an example:

SLA = 3 days - (duebydate - 3/9/09)The call is resolved in 2 1/2 days. A week later the requestor re-opens the ticket - the dueby date remains 3/9/09 - and now the ticket status = open - therefore the SLA is violated. I have goals set up for the helpdesk team which involves the percentage of calls that are resolved within the set SLA timeframes - these types of scenario's are blowing our standards.

Any ideas??