I've tried restarted publisher SQL Server but still the same. It shouldn't be the cache issue then.

The first option looks not good as MSDN said:

Important:

This stored procedure has been deprecated and is being supported mainly for backward-compatibility. It should only be used with Microsoft SQL Server...

Fixed by manually run the below script at distributor server:

update msdb..MSdistpublishers

set active = 0

where name = 'publisher server'

Then run the sp_dropdistpublisher to drop publications successfully at distributor server.

This morning, I can truncate this table now. I've check the sp_who2, it shows:

SPIDStatusLoginHostNameBlkByDBNameCommandCPUTimeDiskIOLastBatchProgramNameSPIDREQUESTID

6 BACKGROUND ...