Personally, I do not like working with spells or black magic. Unfortunately, it does come up now and then. Mostly it comes from a past lifetime.
Find out who put the spell on them, when, and why.
See if an apology is needed from your client to the person who put the spell on them. Sometimes that works.
Other times you have to talk to and convince the person to retract the spell.
If not, then just try to return the spell to them or send it into the light.
If you cannot get rid of the spell, then you need to call in Archangel Michael, Jesus, or God to help you remove the spell from your client.
Practice
Write the four moves on one card — find out who/when/why, ask for apology, ask them to retract, return or send to the light — and pin it above your desk. If none of that works, call in help.