Hi Alex,
welcome at Stridaforum! 
I guess it is a defective freewheel mechanism which annoys you.
That woul be part number ST-340-05
https://stridacanada.ca/product/strida-freewheel/
There’s a “special” tool required for removal, please read the description at Bill’s page or refer to my tool guide, tool T.
Kind regards,
Chris