Though it does not contain an explicit procedure for expulsion, according to an analysis by Just Security NATO member states can by unanimous agreement, effectively expel a member state for material breach of the treaty:

Should the conditions for the existence of a material breach be satisfied, NATO’s member states would be entitled, by unanimous agreement, to suspend the operation of the treaty in whole or in part or to terminate it either in their relations with the defaulting state or among them all (Article 60(2) of the Vienna Convention). For these purposes, a unanimous decision of the North Atlantic Council, excluding the defaulting state, would suffice. No further procedural requirements apply, including those laid down in Article 65 of the Vienna Convention.