I hate it, it ruins newcomers chances of making money in a legit way. It's also stopped me from taking part in races, if I had the option, I'd kick or abuse people putting firetrucks at certain points of the "all around SA" race. I know even admins enjoy doing it, but I still don't take part.
If I'm playing and people are planning a race block I'll just go and attack them with a rustler or something to move them.
Obviously people say "well its a DM/freeroam server, you should be able to do anything" but completely ruining peoples races that was just sucks, unless the race had multiple routes, blocking just totally spoilts the fun for a lot of people.