Fungus gnats are controlled in several ways. Immediate help can be gained by the use of Bacillus thuringiensis Israelensis, a biological larvaecide very commonly available under different brand names. Look for Bt-I.

It might also be necessary to change or amend your potting medium in order to provide something very porous and fast draining. Fungus gnat larvae find it very difficult to survive in such a medium.

Monitoring the moisture level is important, too. Fungus gnats prefer a moist, peaty environment.

Yeah your right the draonage is a real issue, they were sold in decritive pots with no drainage holes so I think they decided to grow the areca in the most soil retentive compost they could, ive had a pair over a year and there drainage has always been abig issue, ive reported them in better pots but the root ball is firmly holding to the soil and believe theres no way to remove the soil from the roots

Fungus Gnats, Sciarid fly, can be most easily controlled by controlling the moisture level in the potting soil. The larva of Fungus Gnats live on the moist organic matter plants grow in, so allowing that potting soil to dry out some will reduce that number of flies you see. Most all entomologists, people that study insects, will tell you that.

Here is a link that might be useful: Fungus Gnats

Palm, you've hit the nail on the head! I think that growers are using dense coir to grow very young plants in, and roots simply do not want to let go of that stuff no matter what.

Though some people repeat over and over again that controlling the moisture level in the soil will solve the problem, it is more complex than that. It is more of a problem of the kind of potting medium than it is moisture content. In a peaty, dense mix the fungus gnats are still happy unless we dry it out to the point that the plant dies. Yes, these are insects that like very moist conditions, but will also do just fine in a mucky potting soil with just enough moisture to support plant life.

Sometimes, we can carefully cut into the root mass with a sharp knife and loosen up the constricted roots, or soak the roots for a short time so that the old medium softens and can be pulled away. If that doesn't work, just doing some root pruning can encourage a flush of new root growth upon repotting into a fast draining medium.

Adding a large percentage of perlite or grit to your potting soil will greatly discourage the adults from laying eggs and won't support the larvae's feeding habits. My potting medium is comprised largely of bark fines and very little peat moss. I haven't seen a fungus gnat in many years, except in nature.

I use mine straight out of the bag with no amendments, but the brand can be difficult to find, especially in the UK. Others make their own potting mix with assorted ingredients...or add to their own potting soil with a large percentage of ingredients that will improve its porosity.

You might consider posting your problem on the Houseplant Forum, where you are likely to get many ideas about how to repot your palms and how to create a good medium.

Thanks for the advice I posted on houseplant for further tips