How can you stop post nasal drip which causes vomitting. ?

Depends: Check with your doctor to find out where the post-nasal drips come from (sinusitis? Sometimes the sensation of post-nasal drips may well be from reflux onto the throat. One can only treat it when the cause is known.

Post nasal drip: Post nasal drip may due to rhinitis, sinusitis, gastro-esopahageal reflux, allergies; find the cause and treat it. For example allergies can be treated with antihystamine( loratadine, benadryl, (diphenhydramine) cetirizine), reflux can be treated with antacid (ranitidine, omeprazole, lansoprazole), see a doc to find the cause.