This is a fully general argument against banning anything at all. Nevertheless, we ban things, and moreover, we do not seem to act as if act of banning is meant to be pointless charade.
We ban smoking in many public places, we ban possession of weapons by felons, or carrying them without a license in many places. We ban selling alcohol to minors, and we ban kids from driving a car.
These measures are highly successful, they resulted in great reduction of the banned activity, and most people would oppose lifting this bans.
In other words, the answer to the question “how banning porn would work?” is “very effectively, like most other things that government bans and actively enforces”.
Likewise, on the internet, for a ban to be effective, heavy handed measures will have to be implemented. For example, a legal apparatus for effective policing and enforcement will grow in power against citizens, edge nodes of a country's internet border will be monitored, personal information and identity will be stored and tracked, etc. The internet has many uses so even if you do not like porn, I think a porn ban specifically will adversely effect people's free speech rights. That is not necessarily true for other types of bans.
If you place restrictions on possession of material, it would have free speech implications that I don't think are trivial. Sure, we don't have absolute free speech but then there would be a debate about what porn is and the worst part of this is there is enough porn out there already that a lot of people would become felons once banned. It could even be weaponized to destroy websites where users can host content. Porn detectors would probably have to scan everything and there would be a non-zero false positive rate.
If you use the internet to restrict it, you're creating this powerful device the government has to censor the internet. I think this last one is something the government is very very interested in having. It's just hard to see how any of this would improve people's lives even if you are totally opposed to porn.