I am able to replicate this issue and made a bug report.
As for the following question.
Is there a way to delete the old poll data for a question in the database?
“The poll is created from results. It is possible, if you know how to use phpMyAdmin to delete the result from mlw_results table in the database. The table name will be slightly different because of the database prefix.”
At the moment, QSM does not really delete results entry from mlw_results table, it is only marked as delete. Customer can physically delete the result entries from mlw_results, if he is confident to do it.
NOTE: If you are not sure on how to do it, please wait for bug fix update, as deleting wrong entry from wrong table can cause irreversible damage to WordPress.