Is there a was to make a firewall rule that avoid mobile devices?
You can create Firewall Rules up to 4kb in size. With that limitation in mind, you could create a long list of possible mobile user agents to block or challenge, but I doubt you’d be able to cover everything, unless you have specific models you want to avoid.
If you are on a Business or Enterprise Plan, however, you can create Firewall Rules with regular expressions, and you could create one to match most mobile user agents.
This support article has some regular expressions used to identify mobile visitors:
The only way you can differentiate mobile browser from desktop is via
user agent (UA). You can define page rules based on UA but there is a solution on Cloudflare for mobile redirection (you can do the same by CF workers). Clients can change their UA so there is not a total solution for this.
You could redirect mobile traffic to a 500 page with no tracking and no content.