Hot knife sounds good. I've not tried these small-scale planes, but recognise the problem from small pieces on runners in bigger kits. The problem is the clipping will cause stress which passes into the part and finds the weakest place to be released. My solution is to use a very sharp knife instead, and for thick joints whittle away near the part to create a narrower join which can be sliced, or indeed clipped, successfully.

You could try this by first cutting away the join at the base. Once the support has been detached this makes the joint at the part less vulnerable.