With the new race system, people can make thier own races using the "yrace" script which is available at the SA-MP forums. however you will need your own server to host it on etc etc.....however RoBo or I may provide one to create races on in the future, however things havn't been thought that far ahead yet, so just wait and see what happens.
Issues with creating the race using the yrace script is that the new race system is NOT based on yrace, and does not use the same file structure as yrace does, which means that ONLY checkpoint locations are carried forward when conversion takes place, the rest of the race options are entered into the converter when that takes place (converter will not be publically released, as ALL races will have to pass through RoBo just like maps are now).
Antijacking at races is a good idea, not something I have looked into, and I might start poking around with that and see what I can come up with, shouldn't be too hard to shove in. Blocking and "shoving" at races, thats slightly harder to control, I think any attempt to code a fix for this would probably result in players getting unfairly treated by the script.....as its not exactly the easiest thing to detect (especially when things have to kept as efficient as possible).
@ FreddJ, a forcefield would have to be controlled by a timer, and the timer would have to be at a super small interval to acctually detect a speedhacking player before they crashed into the person who was racing, which would be incredibly inefficient. Also, if a player had joined a race, then drove away from it to somewhere ike //da and just drove about, loads of players who were not blocking his way would be pushed out of the way, which would be a problem. So in conclusion, forcefield is NOT a practical solution.