Resetraint of trade is an agreement between an employer and new staff member that forbids the staff member to use any skills learnt at the old company. They are quite common.

Part of the reason is that, say I worked for a headhunter, regardless of whether they actually taught me my skills, they do not want me to be in competition with them once I leave.

https://en.wikipedia.org/wiki/Restraint_of_trade