Obviously they are taking you for granted....not what I would call friends....
If you don't want to upset them or lose their friendship then I say put up with it......but if you have come to realise that they are just using you & talking advantage then tell them to either collect there stuff this weekend from your front lawn or bring a tarpaulin & cover it up & leave it at their own risk (it wont be there long thanks to the locals

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My friend pays approx $150 a week for storage...you could ring around find out how much it would cost then & then offer your garage cheaper...just a thought