You probably don't want to hear this, but...the most obvious solution is to get some fins with a slightly larger foot pocket.
Finding a drysuit boot + fin combination that works is not a trivial task. There's no substitute for taking your drysuit boots to a local shop and shoving them into the foot pockets of various fins. Paddle fins that work well with rather large drysuit rockboots include: Scubapro Jetfins, XS Scuba Turtle fins, Hollis F1 fins, and OMS Slipstreams. I'm not as familiar with the splitfins that work with drysuit boots. They are out there, though.
If the fin foot pocket lacks drainage holes, it's possible that suction is being created when you jam your boot into the foot pocket. Drilling some drainage holes in the foot pocket might be helpful in this case. Some divers have had moderate success with spraying some silicone spray inside the foot pocket to make it easier to remove the fins at the end of the dive.
Could you tell us which brand/model fins and drysuit boots (attached or separate) you are currently using?