Guinan's weapon was set at it's lowest setting, & pretty much all incidents of such, the shooter knew the setting of their weapon & the integrity of the hull they were shooting at. To our knowledge, Data knows neither of those things, & the last thing Data's weapon did was thisWhy not? Guinan once fired an energy weapon at the ceiling in Ten Forward. And there have been lots of other times people have fired energy weapons at bulkheads on ships.

It would be a hazard to fire that thing indiscriminately.
You have absolutely no basis to assume that from what he said. Field & force-field are synonyms. It is a field that exerted force against the propelled body of Data, & repelled him halfway across the room. One can at least postulate that inanimate objects might respond the same way. After all, Data is comprised of non-biological materials. So you're saying that Data could remove his arm, thus making it an inanimate object, & throw it at Fajo & that will breach the field, but attached to Data it cannot? There is no grounds for assuming that & it doesn't even make any sense, & even if it did, why wouldn't he just throw stuff at the guy from the beginning, so he could successfully escape? What would the point even be of a defense field that protects you from being touched by someone but not protect you from a projectile assault?Actually, it isn't a forcefield. Fajo describes it as, "It's a proximity-actuated field that impedes positron flow. Not good for your brainpaths in the long term." So if Data picks up a container or something and throws it, it will hit Fajo just fine.
Assuming we overlook my last point (Which I haven't) So you're saying that in the second or two that it takes me to say "Computer put a containment field around Data" Data will have the necessary time to maneuver about this room & find something to launch at Fajo which will knock him fully unconscious fast enough to prevent him from saying it, BUT not kill him?And in the second or so that it takes you to say that, Data would be able to pick up some lose bit of equipment and throw it at Fajo's head to knock him unconscious.
Data's reflexes are certainly fast enough.

He's easily upwards of 10 feet from anything. His reflexes may be good, but even though his dexterity is shown to be miraculous, his maneuvering speed in an emergency is barely above normal human speeds, as evidenced in his escape from the Crystalline Entity attack, where he had to run back to retrieve a child from from Riker
The only definite solution in this moment for Data to remove all the potential threat that Fajo represents, is to fire that thing & hope it's fast enough & strong enough to breach that perimeter field