If you go to Admin>Config>Formative Assessment Statements -
And using the Subject Component area -
Choose the subject you wish to delete the statements from, choose the statement and click on the Edit button. Here you have a choice to delete the statement you no longer require.
Please be aware though - If you choose to do this, then you will lose all historic information linked to those statements. You can choose instead to rename the statement if a similar one is needed. If you do delete these statements all linked marks will be lost.