How do I delete all calendars in SPlanner on Samsung S7

I keep my main calendar in Outlook, but we use Google calendars at work. Since the latest firmware update, my S7 has imported all my Google calendar appointments as well as all those of my colleague, on top of those already imported from Outlook.

I just want to delete everything in SPlanner and start again. I could do it on my old S4, but not on this S7

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