Firejail  is a very easy to use piece of software, initially developed to make Firefox more secure by isolating it (putting it in a sandbox) from the rest of your system. Firejail has developed beyond that & can be used simultaneously on many parts of your system. It is worth reading about on Firejail site (linked to above).
From its Github page:
Firejail is a SUID sandbox program that reduces the risk of security breaches by restricting the running environment of untrusted applications using Linux namespaces, seccomp-bpf and Linux capabilities. It allows a process and all its descendants to have their own private view of the globally shared kernel resources, such as the network stack, process table, mount table.
Firejail is available in the repos. Install it using your graphical package manager or with pacman.
sudo pacman -S firejail
Simply prefix your command or application with firejail. Example:
You can even integrate firejail into your whole desktop. Every application you launch will use the firejail sandbox if it is supported by default. To do this use the command
For a list of all supported applications see here
The firejail team also develops a graphical user interface for firejail called firetools. It is available in the Arch User Repository.
Block an application from accessing the internet
You can use the option –-net=none to ensure firejail does not allow any network access like so:
firejail –-net=none thunderbird
But that option will block local network access, such as access to shared folders. To still have local network access, but block the application from accessing the internet use option --protocol=unix. Example:
firejail --protocol=unix thunderbird