So again, with the parkour i'm doing, i was wondering if there was some command to make it where: when a player touches a certain block (like the ground), they are sent back to their spawn point. 

Two options.
1) Turn on fall damage with map death with /map death (or /os map death)
Pros: It makes long falls anywhere in the map deadly without extra work
Cons: It is very glitchy and can easily be survived if you just hold jump
 

2) Build deadly areas using a temporary block (say, sapling or something). When you're finished coating the area you want to be deadly in saplings, do /replaceall sapling nerve_gas. Now the area is filled with deadly invisible blocks that send you back to your spawn.
Pros:
• Players can't use any exploits to survive it.
• You fully control which areas are deadly regardless of fall height
Cons:
• If you miss covering an area, people might survive in areas you didn't want them to.

tysm!