Currently, custom entries cannot be removed from the list of available entries under WI customizations.
Once a new entry has been created for an attribute in ARS Web Interface under customizations, it will be available in the list displayed under: Form Editor | Add Entry | Select.
Currently it is not possible to remove the entry from the list if you no longer need it. Even if an entry for a virtual attribute that was deleted in ARS, it will still appear in the list.
An enhancement request (TF00426118) has been created detailing the feature.
The product team will evaluate the request and this feature may become available in a future release of the product.