Tripping will work, if they aren't flying. The Improved Trip feat can be very useful, if you have the right weapon and build.
When facing an opponent with a hard-to-hit AC, the feat/maneuver lets you make a touch attack to initiate the Trip, followed by the contested Trip Check. Unlike many other special maneuver checks, this one is based on size and strength, with a bonus for the feat, bit most pointedly, it ignores BAB. Once you Trip them you get a free shot at them while they're prone. Their AC drops by four when prone, so can hit things you might otherwise not be able to.
Now I'm not sure what "Lock Down" means, but you can't effectively restrain most Demons. Many have Improved Teleport, at will, so they can pop right out of any hold, lock, or restraint there is.
It also means they can use a Standard Action to get to their feet again after a Trip, even moving to an advantageous position relative to you. That denies the Tripper (and any other adjacent allies) the Attack of Opportunity people normally provoke when rising.
One common mistake with the Trip maneuver and that AoO is that you can't use the AoO to Trip them again, and thus keep them down indefinitely. The AoO happens when they start the provoking action, so they're already down when you take it. And you can't Trip someone who's already Prone.
Weapons that can be used in a Trip are the Whip (almost useless for anything else, in fact), the Spiked Chain (Exotic weapon), Flail (including Dire and Heavy flails), Bolas (trip at range), Guisarme and Halberd (pole arms), Gnome Hooked Hammer, the Kama (Monk weapon), the Scythe and Sickle. And, of course, you can attempt it as an unarmed strike as well.
What all that means is that it's a readily available maneuver that can be accomplished with a wide variety of weapons, one handed, two handed, reach and at range, standard, racial and/or exotic.
Now, Trip is fall less effective against opponents who are larger than the one attempting the Trip: +4 in the contested roll for every size category larger, and +4 if they have more than two legs, or are "unusually stable". (Dwarves are apparently "unusually stable", per the trip rules on page 158 of the PH.)
The defender can also opt to use Dex in place of Strength in resisting the Trip attempt.