I FOUND A FIX to this issue -- both at account registration as well as contact us page.
NOTE: avoid using many apps ........I notice more apps you use...the site becomes very very slow, all can be done in code.
I am running my own firm axon.com.sg for over 2 decades and managing few clients in shopify.
I try all modes and even apps...it does not work. The spam originates from one source yourdomain.com//account/register
what i have done is to edit the source (need tech knowledge please..not for normal users), for current and all other themes, set as protected. it would not affect the overall eCommece as client can register at checkout. Need assistance email firstname.lastname@example.org or visit www.axon.com.sg . Also whatsapp me online at https://www.axon.com.sg/whatsapp
Note: This mode require some techie guy to edit..simple edit but effective. I tested on few sites...from every 5 mins 1 spam entries...to basically 1 per day or none....excellent. We can't stop spam 100%...but if we can stop 99% ..it is perfect.
There's no code that you can add to the theme to stop bots from posting directly to the endpoint. So for simple bots, those mods can help but for anything more sophisticated it will rely on the Shopify team to refine their bot protection.
What makes your method more reliable than others?