Dear Prashanth,
You should understand the underlying principles of transactional analysis when you have to interact with others particularly union people with a negative mind set towards anything proposed by the management.
Union office bearers would behave the way you mentioned so as to make their presence felt by others; in such a situation, you should not react but respond to them in a courteous manner so as to prove that your point is appropriate to the issue on hand; in stead, if you engage yourself in argument, the main issue would be sidelined and culminate in confrontation or an IR issue as you mentioned.
Whenever you feel that things may go out of your hand because of the intransigence of the union people, better you should seek the advice or interference of your superiors.
In a negotiation, when the union office bearers take up an unwarranted issue vehemently, you should not tell them that they have no locus standi to raise the issue even it is so. On the contrary, convince them the possibilities of the things going just opposite to their expectations. If you conduct yourself in a dignified and persuasive manner, they will automatically cool down.