linux-image-2.6-686 is not the actual kernel image. It is a transitional package which has a dependency on the latest 2.6.x kernel-image. So your kernel will be upgraded everytime you do an upgrade. If you don't want to have your kernel upgraded automatically then install the actual kernel-image such as linux-image-2.6.18-3-686 and remove linux-image-2.6-686 / kernel-image-2.6-686. This is the same for smp kernels.