I got a replacement spoke from a local bike shop but it looks like its a bit too long... how would you cut it to the right size?

You wouldn't cut it. A spoke has to be the right length within a couple of mm. If it is visibly too long, the threads would bottom out before you could tighten the spoke.

Unless you have a very expensive spoke cutting and threading machine. Just get a new spoke or try to exchange it.

Bike shop can sell you the correct length or if they have a spoke cuttung/threading tool they can cut it to size.

this is not the case here... I can install the spoke but as I tighten it it starts to become too long and is crowds the tube... enough that it will pop the tube...

Do you have single wall rims or something?

That means your spoke is at least 5MM too long.

If you have enough thread to get the proper tension on the spoke, you can file off the excess protruding through the nipple.

But if the nipple bottoms out on the thread before the spoke is properly tensioned, you need to get a shorter spoke.

See Bill K's post.

The case you have is that the spoke is too long, get one the correct length.

The threads on spokes are rolled, not cut. Which means that the threaded part of a nipple cannot be threaded past the end of the threads on the spoke without damage to the nipple (low to no spoke tension). Most nipples have a counter bore on the end pointed towards the hub before the threads start. If you try to push the unthreaded part of the spoke beyond the counter bore -> scrapped nipple.

Are you able to get the spoke to the proper tension with the wheel true? If so then just use a file, grinding wheel, or dremel to grind off the excess length that sticks out past the nipple.

But if you run out of threads before the spoke is at the correct tension then you'll need to get a shorter spoke.

remove the long spoke, and a correct length spoke from the sam side of the wheel... take them both to the shop that sold you the long spoke, and get the CORRECT length. THEY screwed up, not you.