I have an old trek road bike, and last time I went riding, the pedal pulled out of the crank and stripped it, so now I need new crankarms.

I have a triple tiagra hollowtech crankset, and if I could use this, I wouldn't have to buy anything. I was wondering how I could make this into a double. I could remove the small chainring, but then wouldn't the chainline be not as good as before?

How would I get a better chainline? Do I have to buy a part, remove a spacer, or is there no way to do it?

You can remove of the inner ring and the chain line will be exactly the same as it was before, so in a way it wouldn't be worse. Depending on the frame, the Hollowtech BB may have a spacer between the outboard bearing cup and the frame. If it does, you can switch possibly switch the spacer on the drive side over to the non-drive to position the crank a little closer to the frame.

Hollowtech triples have a ~1/2" spacer on the drive side. You could move it to the non-drive side, but it looks really bad.

hollowtech is also known as octalink
hollowtech II is the newer outboard bearing.

If it's an older bike, it should have octalink which still uses different BB axle sizes for double and triple.

It is possible to put new threads into a stripped out crank. Ask around at your LBS to see if they can do it. I'm pretty sure there is a heli-coil type of insert that can be used for this.

Otherwise, as mentioned above, if you have an octalink system, the triple BB is 118.5mm and the double is 109.5. If you put a triple crankset on a double BB the crank arms might hit the chain stays. If you get a triple BB you will have the correct chainline - for a triple. The triple chainline spec is slightly different than the double chainline spec but in practice it probably won't matter as long as your FD can reach the outer ring.

If it's Hollowtech I (octalink) you can maybe use a bottom bracket with a shorter spindle.