Drink pomegranate juice and eat some ginger 20 minutes before. That stops any nausea with Ayahuasca for 6-8 hours, it evens "fix" the horrible taste to many.
I think it’d be counterproductive to stop the nausea, when nausea is an integral part of the Ayahuasca process of letting go, in addition to crying, yawning, sweating, etc. I just listen to my body and let it do its thing; sometimes I need to purge, sometimes I don’t need to purge.
It stops the nausea, but you still can purge (can't tell why). But it feels that you are just cleaning your body. And without nausea you can listen, IMHO, to your body, more carefully
I never imagined being able to vomit without the nausea, so that’s pretty interesting hah
YMMV, but for several friends and me, the urge comes all at once and the purge happens. It's like an "immediate" nausea.
The purge isn't necessary for healing, shamans use your physical purging as an anchor to help you emotionally purge, but that's just another technique.