Can’t Param Purush stop the creation of this material world by allowing Niranjan to return to His abode?

No, Niranjan received boon to rule for a period of 17 Asankhya Chaukri Yugas. Till then he can’t go to Amarlok. The word given by Param Purush can’t be broken.
Niranjan himself makes this attempt. He prays to Param Purush after every great dissolution to take back the Souls and allow him to return to Amarlok as he has grown sick of creating the universe again and again. Sahib says that He has granted his request to rule for 17 Asankhya Chaukri Yugas and as such, he should continue to do the job of creation as before.