NALC model standing orders 1 (a):
Motions on the agenda shall be considered in the order that they appear unless the order is changed at the discretion of the chair of the meeting.
If you have adopted NALC MSOs unamended, the chair has discretion to amend the order of the agenda.
Another course of action could be to pass a motion that the member 'not be heard' (MSOs para 10 (a) xii) for the duration of that motion (due to the assumed pecuniary interest) and allow the motion to progress through discussion to vote excluding that member if the vote for them to be 'not heard' is passed.
Another option could be to proceed anyway and submit a code of conduct complaint in relation to failure to declare or absent them self after declaring an interest. If they don't even declare the interest, then the complaint would be on that basis alone.
Which course of action would be most appropriate would have to fall to the judgement of the chair really and would be dependent upon the nature of the motion for which the member has refused to declare / leave the room after declaring an interest.