Not sure where these myths originate but used coffee grounds will do nothing for whiteflies. Or any other flying insects. Insecticidal soap or Neem oil are good choices for control but until you control both adults and their eggs, this will be an ongoing problem. especially in a greenhouse or solarium.

You can try hanging yellow sticky traps, sold at most garden centers and where they sell pest controls. The flies are attracted to the yellow color - they think it is new foliage - and when they investigate, they become attached/stuck to the traps and die. These used in combination with the more traditional insecticides should do the trick.

Used coffee grounds don't deter slugs either. It is a persistent gardening myth.

Thanks, guys!