As has been mentioned as a comment to your question, all you can do is make it harder.
Current WiFi protocols require that the client and AP both know the PSK. This is unavoidable as it stands, and I am not sure of any plans to change this.
You can employ a number of different techniques to make secrets harder to obtain. You need to take into account your threat model though - what are you protecting, who are you protecting it from, and how long you want it to be protected for.
The task is not dissimilar to protecting the keys in a hardware security module. These often use advanced techniques to prevent someone - even with extended physical access - from getting secret keys out.
Specifically, for most IoT devices, they are already stored in a protected area i.e. inside the house. The Ring Doorbell and some CCTV IP cameras break this barrier and are in areas where an attacker can more easily access them - consideration should be made for this. That said, you should not rely on everyone in that protected area being benign - for example, if your teenager can only use a time-restricted SSID, they could try and recover another PSK for a normal SSID from another device.
Obviously you should protect the key being read out over the network, as we found in the Ring Doorbell (full disclosure, I am an employee of Pen Test Partners who found the issue). A number of common WiFi SoCs/boards come with SDKs that have holes such as these though.
Some WiFi SoCs/boards allow the PSK to be set once, and then make it hard to recover the key. At a bare minimum, these require connecting JTAG or SPI to read some memory. At worst, you may need to develop new attacks to recover the content of onboard EEPROM.
Physically potting boards makes accessing buses and chips harder, and will deter attackers.
This could be extended further to add anti-tamper functionality such as that found in hardware security mechanisms or Chip & Pin terminals.
Encrypting the key with a symmetric key is largely pointless in most small embedded systems that can by physically attacked.