...besides "staying slow on their way".
Well, I hadn't really noticed these tactics before because they were all natural to me. Came from Driver 1 (PS1) in Pursuit races, and have seemingly been working in RR3 all these years.
Tip #1 is: blip the brakes a few times to slow a bot immediately behind you. If bots see you as an obstacle, it is natural in them to release the gas to avoid contact or steer a bit to try another line. Either way they tend to slow down in RR3.
Tip #2: choose your line accordingly. This one includes'staying slow in the exit of a turn'. By the way... by doing that you get a nice feel of the exit speed of a given bot and can regulate your speed throughout. I gather that the slow turn-exits are where you can block the bots more effectively.
Tip #3: maybe the most important in current team event, jump straight into top positions before you let some bots pass you and hold the position you want. Looks like by reaching the leader and fighting it side-by-side for the number 1 spot, you indeed slow down everyone behind you. Once you let a few bots pass you, the remaining bots are still slower than originally, so it's easier to control them. A restriction to this trick is when the number 2 or 3 bots get much slower as well, so it might be hard to keep, say, number 4, without being turned around or passed be others.
Hope this helps