Maintain stable data in a database
I don't really have the possibility to try jMeter now, so I don't know how following the below advice would affect your tests.

Normally benchmarking is done so that after the tests the original database state is restored.  This is achieved by rolling back all changes.

In the case of jMeter you have to set AutoCommit to false in the JDBC configuration, then issue a ROLLBACK as the very last command in your test script.