Iit is not very hard to really remove all .deb files:

find /sw/fink/ -name "*.deb" -exec rm \{\} \;

does that ;).

So yeah, we could build such a thing into "fink cleanup". Similarly, something that deletes *all* source archives in /sw/src -- and for that matter, also any residues in /sw/src/fink.build

So I see no technical problems with that.