I Know it is bit late, but may help someone in future.

Delete related publication entries from following tables. This will clean up Replication monitor. Please make sure you delete the old publisher(which doesn't exist any more) and not the current one.

select * from distribution..MSpublications

-- delete from distribution..MSpublications where publication_id = **

select * from [distribution].[dbo].[MSsnapshot_agents]

-- delete from [distribution].[dbo].[MSsnapshot_agents] where id = **