To remove events that you have imported from your calendar or the Reminders app from Structured, open the Structured ⚙️ settings and tap on Calendars or Reminders. Here, you can deselect every calendar or reminders list you no longer want to be displayed in your timeline. You will need to do this on every device you use Structured on, otherwise iCloud will still synchronize the events across all of your devices.
If you only want to remove a single task from your timeline, you can simply hide it.
Tap on the task and select Hide. The tasks will be removed from Structured, but will still be displayed in your calendar or Reminders App. Be aware that you cannot un-hide the event again.