Simple things first. Sometime guy's will almost have a death grip on a BB/DB/cables when curling. Not really needed if you can get by with a looser grip. Lessens the tension on the grip/forearms themselves. Can also apply a false grip (thumbs under the bar). More like cradling the bar than having a grip (tight or otherwise).
If trying to reduce the influence (or as much as you can) of the forearms in a curling action I might suggest the following: close grip BB (or E_Z bar as second choice) curls. This would be very close grip, about 4 to 6 inches apart. Have the elbows braced against the waist if standing. Can also do these off the end of a bench, with the elbows pressed against the inner thighs or resting the elbows off the end of the bench (probably could use a folded towel for the elbows ). Too the extreme, try extra wide grip BB curls also. Even drag curls focus on the biceps very well. Another exceptions biceps builder are close curl grip chins, all the way up to the upper pecs. If done right you will feel it strongly in the biceps.
With DB's you may consider incline curls. Start from the bottom position, with the thumbs facing forward. As curling up, slowly twist the thumbs/wrist inward (one of the functions of the biceps is to twist the wrist outward). At the top position the thumbs should be pointing away from the body. Should also feel a bicep cramp when doing this. Actually any pure BB'ing type DB cirling is done like that. Either way, get a better focus on the arms. Scott/Gironda bench curls, DB or BB is also very good.
Lot of other ways, like leaning into the bar, elbows high, when doing regular BB curls, etc. In any event, just have to do a little self exploring to see which method may work the best for you. And you only. Good Luck.
