DON’T TRY TO PUT IT OUT WITH WATER. If you don’t know why check out this Worldstar Hip Hop video.
A flare-up is a grease fire (usually in miniature). When you’re grilling with gas, some of them are going to happen.
To mitigate flare-ups, try trimming fat before grilling or using aluminum foil.
Flare-ups are not inherently bad, or unusual. If you get one that won’t go out, your best bet is to let it burn itself out (see above).