Those objects are on the "Page Background" layer, which is locked by default (so you don't accidentally move background objects). So you need to unlock that Page Background layer in the Page & Layer gallery, then you can select those objects and either remove or resize them to fit the narrower page.
However I'd expect those background objects to have been reduced in width automatically when you created the Mobile variant, so we'll look into why that hasn't happened for your design.
Thanks
Neil
Xara
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