There is an option to remove the Silk Central built-in reports (Test Manager reports as well as Issue Manger Reports).
Locate the following 2 files in the sub-folder "StandardReports" in Silk Central's installation directory (e.g. "C:\Program Files (x86)\Silk\Silk Central 16.0\StandardReports")
These files contain the built-in reports for Test Management (TMReportFilters.xml - available via "Reports / Details View" in the main menu) and Issue Management (IMReportFilters.xml - available via "Issues / Reports" in the main menu).
When a database is connected in Silk Central, then the built-in reports are first removed from the database and then those reports are added again that are specified in the two xml-files.
Therefore, to remove the built-in reports, they must be removed from these xml files and the database reconnected:
<?xml version="1.0" encoding="UTF-8"?>
<reportfilter name="Global Reports Root" description="null" defaultrenderer="-1" askforrenderer="false" module="1000" timeout="0" builtinid="TM1">