You can delete rectangles representing block instances. However, this is the same as deleting block instances from the Blocks tab.

To do this, select the block instance’s rectangle, and then press the Del button. If that block had any wiring, either as a source or a destination, it will be removed.

Warning: If any rules’ condition or action parts refer to any feed, field, parameter or operation of the block instance you have removed, the references will be reverted back to their non-terminals. This will make those rules, and therefore the enclosing states and the scenario, unfinished.