Unfortunately, this cannot be avoided. It is possible to change the net order price in a PO item after the GR has been posted.
What you cannot do is add new delivery costs. This is forbidden with an error message.
In the customizing transaction OMF4 you can disallow the changes for the net price setting it as a "display" field only for the field selection groups ME22 and ME22N. Please, take into account that AKTV, NBF, PT0F will also be checked (or other entries depending on your specific document type and item category).
Unfortunately, this will affect the "net price" in the transactions ME22 and ME22N all the times (this is, the field will be a "display only" field always; it will not allow changes, independently of whether you have posted the goods receipt or not).
I hope this helps!