No especifica su RDBMS, así que aquí está en MySQL
DELETE teammembers FROM teammembers
JOIN teams on teammembers.teamId = teams.teamId
WHERE teams.eventId = 1
No especifica su RDBMS, así que aquí está en MySQL
DELETE teammembers FROM teammembers
JOIN teams on teammembers.teamId = teams.teamId
WHERE teams.eventId = 1