I don't think you can. Individual programs can limit how much IO they do; jb_fvwm2 gave an example for rsync. There is no OS-wide facility for limiting/throttling the IO of a particular process, nor is there an IO priority system and scheduler. For CPU usage, there are such things as priorities, the nice command, the whole mechanism behind it, not for IO.
But let me ask another question: Why do you care? What's wrong with tar using 100% of the disk? Matter-of-fact, you have to remember that spinning disks are one of the few things in the universe that get more efficient the more overloaded they are. From an overall throughput standpoint, you really want your tar to run as fast as possible, even if that's an inconvenience for other processes that use the same disk.