Every practitioner eventually meets a client who wants a treatment they should not have, and who is persuasive about it. Deciding where your lines are in advance makes holding them straightforward.
Absolute
Pregnancy and breastfeeding. Active infection in the treatment area. Keloid scarring history. Pacemakers and implanted electrical devices. Certain photosensitising medications.
Isotretinoin
Current use, and a clear interval after stopping, in line with prescriber guidance. This is not an area to improvise in.
Relative, and requiring judgement
Poorly controlled diabetes, autoimmune conditions, recent sun exposure or sunbed use, and unrealistic expectations — which is a genuine clinical contraindication rather than a commercial inconvenience.
Recording the refusal
Note what was requested, why you declined, and what you advised instead. A declined treatment properly documented protects you. An undocumented one is simply a client telling people you refused them.