The solution

Coding the solution involves these steps:

create a class variable to hold the DAO;
use the DAO to retrieve the instance of the entity;
instruct the entity instance to cancel its data on the database.

Create a class variable to hold the DAO

This step is identical to that in You want to read some data from a database table above.

Use the DAO to retrieve the instance of the entity

This step is identical to that in You want to read some data from a database table above.

Instruct the entity instance to cancel its data on the database

You must code a call for the entity instance to cancel its data on the database:

Figure 1. Calling the cancel method on an entity

// do the cancel, passing the version number from the client
    someEntity.cancel(key.versionNo);

The entity instance will:

perform cross-entity validation, allowing other entities to veto the cancellation; and
cancel its data from the database.

For an entity which supports optimistic locking, you must pass the version number held by the client struct.

Important: Do not be tempted to use the version number on the entity instance which has been retrieved, as this would render the optimistic lock mechanism useless:

Figure 2. Incorrect - bypassing optimistic locking safeguards

/** ********** VERY VERY BAD - DO NOT DO THIS! ********** */
    // do the cancel, passing the version number from the entity
    // instance
    someEntity.cancel(someEntity.getVersionNo());
    /** ********** VERY VERY BAD - DO NOT DO THIS! ********** */