What are you hosting the service in??
The easiest solution is to leave the thing compiled AnyCPU, then write a custom hosting app to host your service, but compile that x86 only. When the service .DLL is loaded, it'll be forced to run 32-bit only.
Another option, which I haven't tested at all, is to edit the resulting .DLL and change the CPU type using corflags from the .NET Framework SDK:
corflags /32bit myService.dll
You'll have to be careful doing this though as the service MUST be hosted in a 32-bit process, whatever it is your using to host with.
If you're using IIS to host, you may want to check out this
], especially the option called "enable32BirAppOnWin64", which is False by default.