coffe works well
freezing does but remember to freeze them for 3 days or more otherwise they will come back to life!
and scalding works aswell. some people think that scalding takes the colour out but it does not seem to matter.
try it freeze 1/2 a pint of maggots for todays then leave them out in the sun most will start wriggling again. then leave 1/2 a pint in the freezer for 3or 4 days. then leave them in the sun and see how many come 2 life. there is a big difference