Block new participations in speakers/companies

[Francisca105]

For companies that were bought, sold or for some reason don't exist anymore
OR
For speakers that already did a keynote (/other conditions) in a last recent edition

there should be a way to block new participations of being added (unless admin/coordination - but it should display the information that the entity is "blocked" from new participations)

It would be interesting to provide a reason for that ("blocking")