I don't use Lightroom, but if you open Photoshop 64bit version before sending images from Lightroom, I am sure it would use the already open 64bit version.
You could also take a look at this link, but it seems a bit of an OTT solution to me:
Why work so hard to remove something you might one day need to use?
How much is your time fighting disk full issues worth? Just buy a bigger disk drive.
At installation you have option to install 64 and 32 or either one seperately.
Did not install the 32 bit version, so can't say if there is a seperate uninstall icon for it.
Rename the 32-bit Photoshop.exe to Photoshop.ex_. Right click a PSD file, select Open With... "Choose Default Program" and change the default app to the 64-bit version. If you ever need to run the 32-bit version, you'll have it, but you'll never have to worry about it running by default.