This is what I've done in the past....
Removing User Created Form (not report) Names in Report Settings
To delete the line that a user created for their form, look in the Sy_reportsetting.m4t file via DFDM. Report setting name will appear as part of the key.
............\MAS90\MAS_SYSTEM\SY_ReportSetting.M4T
Also look at
...........\MAS90\MAS_SYSTEM\SY_ReportOption.M4T
And
............\MAS90\MAS_SYSTEM\SY_LastFormUsed.M4T