https://www.irs.gov/uac/Business-or-...for-Deductions
This link is from the IRS regarding business vs. hobby. The major presumption the IRS makes in determining if you have a business or a hobby is whether whether or not you made a profit in at least 3 of the past 5 years including the year you are filing. Basically the way they look at it is that if you perpetually have self-employment losses and try to use those losses to offset income from another source, they are going to tell you to quit using your hobby as a tax break.
The ideal way to handle tax issues related to officiating is to get a ledger. I keep a log of everything on my ledger with detailed information including the dates of expenses incurred and how the expenses were associated with income when applicable (i.e. if I had travel expenses that were not reimbursed, I entered the mileage in the travel section of the ledger with the date and which game I traveled to in what location). I keep track of all of my income as well and I know by the end of the year what to expect if I do receive any 1099s. Regardless, I file based on the information I have kept for myself and think of 1099s as potential reminders if I do happen to forget something.