If the instructions are followed correctly, there should be no reason to do this but if, for any reason, you decide to disable DMARC protection you have two options:

1) Modify your DMARC DNS record to reporting-only mode.

2) Delete your DMARC DNS record.

The DMARC DNS record has a TTL of 5 minutes so once you make any of the changes above it will complete its propagation in 5 minutes. 

Once you switch it off your domain will no longer be protected until you switch it back on again.