It's generally hard to throttle java - doubly so in this case where the app in question is designed to solely focus on crunching as many numbers as possible. Luckily, the diablo miner has an -f option to dial back the computations.
It's trial and error, but you can read up on this huge thread and search for CPU for more details.
"-f 1000 will greatly reduce mhash speed, but increase desktop interactivity. -f should be a multiple or divisor of 60, lower values increase speed but decrease desktop interactivity, higher values do the opposite."
It's best to get GPU only going as CPU mining is very expensive in terms of heat and electricity as well as reduced performance.