If you’re lucky, the same registry fix as non-Synaptics touchpads works:
HonorMouseAccelSetting (dword, should already exist) to
1. Then reboot.
If you’re not so lucky, you’ll need the MarkC mouse fix. Mirror here:
Installation is self-explanatory – install the .reg matching your screen DPI.
After a reboot, acceleration will be gone. However there is some unavoidable input lag from this approach, as it is effectively inverting the acceleration.