Deleting unused Main Account Values in Ax 2012

A financial dimension value is based on the Main Account  record and has been used on a transaction, You cannot delete the Main Account record. 

In order to get rid off un-used dimension values, delete the related records in the following tables
1.DimensionAttributeValueCombination
By default the ledger dimension values are stored in DimensionAttributeValueCombination.
The DimensionAttributeValueCombination table contains information about accounts and various dimensions combinations that are used. Anything that uses dimensions will hold reference to a record on this table. 
2.DimensionAttributeLevelValue
This table contains the usage of  a dimension attribute value in a given dimension hierarchy.
3.DimensionAttributeValueGroup
This table represents a group of values for the specific dimension set.
4.DimensionAttributeValueGroupCombination
It contains the usage of dimension code groups in a  dimension code combination.
It allows a group to be re-used in multiple combination.
5.DimensionAttributeValue
This table contains the values, such as dimension codes, for a specific dimension.

After removing the reference records in the above tables, you can remove main account from its master itself.