RE: Deleting modified reports using SQL
Modified reports are not stored in a SQL table(s), instead they are stored in dictionary files (.DIC).
Each Dynamics GP product has its own dictionary file, e.g.:
Modified reports for 'Microsoft Dynamics GP' are stored in REPORTS.DIC.
Modified reports for Field Service are stored in RPTS949.DIC.
A typical location for these .DIC files:
C:\Program Files (x86)\Microsoft Dynamics\GP2015\Data\
You can find the exact path for all Modified Reports and Modified Forms dictionaries in the Dynamics.set file (you can open it using Notepad).
As callen suggest, if the Modified Reports you want to delete are part of 'Microsoft Dynamics GP' you will need to delete the REPORTS.DIC file or even better rename it.
If the Modified Reports are part of 'Field Service' you will need to delete or rename the RPTS949.DIC file.
I hope this help.