You can save money and time by taking the airline and bending it in half. This will pinch the airline, decreasing the air flow. You can then tie a rubber band around the pinched area to keep the flow at the desired rate.
This is a really bad idea. I know, I used to do this and spent a fortune on new diaphragms over the years. By far the best way is either to split the flow between two airstones or to buy a 'gang valve' from your LFS or on-line and bleed off excess air.
If you put too much back-pressure on the airpump then the (rubber / neoprene) diaphragms bulge and wear out much, much faster than they should as they are working in a sub-optimal shape which stresses them. The valves also are rubber things that can be prematurely damaged by back-pressure. Of course the best option is to buy a good quality airpump with a rotary electric 'output control' and just dial in how much air volume you require. (Air-stones that are too small or clog will also put too much back-pressure on the pump.)
I have three Hagen 'Optima' pumps that I've had for over 10 years now. They are adjustable between 5l/min and 5.5l/min air output and a maximum pressure of 4.2 PSI and have a washable air pre-filter. They also have really good sound-damping rubber feet. When I bought them, going by past experience with other air-pumps I bought a bunch of spare diaphragms for them. I still have the spares in unopened packets. :happy:
(I am not an employee of Hagen and have no financial interest in them. There could well be other excellent airpumps around but I can only speak from experience.)
One of the things that I learned from my time in the aquarium industry is that back-pressure is the biggest destroyer of airpumps. You can either buy cheap ones, restrict the flow and generally allow backpressure to build up, then replace them when they die or get noisy... Or you can spend a bit more and get a quality unit that will last you for ages and save you a lot of money long-term.
I hope that helps. :happy: