I struggled with nits last summer with Mini Boo, who'd even passed them onto me!
The only way they eventually went was a nightly ritual of conditioning her hair and running a Nitti Gritti comb through it, plus the long 7 week hols, away from school! I also genuinely believe her head constantly submerged in sea water (swimming) whilst we were on her jollies helped too, though that's obviously not a scientific fact.
I also really dislike the stigma attached to nits- I sent Mini Boo in to school with a letter stating she'd nits and would her teacher ask the rest of the class to be vigilante, as far as I know that never happened :-(