Can I obtain a re-entry permit at IT Laksi, and what are the requirements for cancelling an ED Plus visa?

No, re-entry permits aren't sold at IT Square Plaza Laksi

You can get a re-entry permit at the immigration office that issued your extension (where you live) or at the airport before you fly out

To cancel an ED extension you just need the documentation from the school/uni stating when you stop studying, go to the immigration office that issued it and cancel it.