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If you never want a router to be used as a transit path then just set the overload bit in the LSPDUs generated by it, so on R1 in this case.
That would be a hard "you cannot send traffic via me" signal to the two LSRs.
There are various other option available under the overload config depending on your needs (can have it act more like overload in OSPF where it just sets max metrics and there are also option to allow route leaking etc)