Hey Gabriel,
Since Odoo Online doesn’t allow direct customization, you need to rely on settings and potentially some technical workarounds:
1. Disable Guest Checkout from website setting (which I guess you already know after reading lot of frustrating answers)

2. Hide the Signup Option ( below way I think can be workaround)
technical > view modification (I commented that signup-page view)

3. Hide the Sign up redirection on the login page as well . again its view html moditication

these above step - again are not proper way but workaround.
Best way is to take your odoo to the Odoo.sh - to have more customization capabilities. which is obviously needed as Odoo ecommerce is not - ready made for the B2B wholesale.
On top of this , if further need to restrict the existing user - it can be done with the combination of defiing separate access rights and assigin it to all the ecomerce users and reducing the User interface with the help of View html modification can be achieved.
I hope I point you to some workable direction.
Thanks,
Rajesh
ERP Consultant | Wholesale | Manufacturing| Integrations
Silent Infotech Inc ( https://silentinfotech.com )