You can detect the Intellect Devourer using Detect Thoughts. However, you can't target it.

Detect Thoughts states (italics notes mine):

You can also use this spell to detect the presence of thinking creatures you can't see. When you cast the spell or as your action during the duration, you can search for thoughts within 30 feet of you. The spell can penetrate barriers, but 2 feet of rock, 2 inches of any metal other than lead, or a thin sheet of lead blocks you. You can't detect a creature with an Intelligence of 3 (Intellect Devourer has 12 INT) or lower or one that doesn't speak any language (Intellect Devourer knows Deep Speech & Telepathy).

Technically, an Intellect Devourer doesn't speak Deep Speech, and Telepathy is not really "speaking", but that's obviously not the intent of the spell.

However, the PHB states on page 205, in the chapter Spellcasting > Targeting:

A Clear Path to the Target

To target something, you must have a clear path to it, so it can't be behind total cover. If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

This applies regardless of whether or not a spell requires you to see a target.

Ignoring full cover

Since you need to have a clear path to a target to target it, and no AoE can circumvent the Intellect Devourer's total, 360° cover, we need to find an effect that isn't explicitly blocked by total cover (which rules out attack rolls), that doesn't only affect targets within an AoE of the types listed in the PHB, and that doesn't specify that your "target" something. As expected, this rules out most spells or other effects.

The closest that an effect gets to damaging the Intellect Devourer is the Forbiddance spell, which doesn't describe its AOE via one of the methods in the PHB, nor does it explicitly target creatures, and therefore it can circumvent full cover. The spell deals 5d10 damage to designated creatures within the area, such as elementals, fiends, etc.
Unfortunately, this list doesn't include aberrations, so unless your DM houserules that it does, it's of little use to you (unless you don't want the Intellect Devourer to Plane Shift away, which it cannot do anyway).

Aside from the Ghostly Gaze + Sacred Flame combination that David Coffron mentions in his answer, there are a few other spells and features that might allow you to damage the Intellect Devourer. These are detailed in the subsequent sections.

If you stick to the strict interpretation about targeting as described at the beginning of this section, you can also target the Intellect Devourer with spells such as Power Word Kill or Divine Word, both of which are not AoE spells, don't require an attack roll and don't mention a "target" or "targeting a creature". Note that you also need to have the Warlock's Ghostly Gaze feature, since you need to be able to see the creatures.
However, if you apply the common english meaning of "targeting", the single creature affected by a Power Word Kill is most certainly "targeted", and so are the (albeit potentially unlimited number of) creatures affected by Divine Word.

Geas

If you cast Geas on the Intellect Devourer before it enters the body (it cannot be targeted by Geas once it does), you can deal damage to it when it acts "in a manner directly counter to your instructions". Of course, this is a rather constructed example, since it takes one minute to cast Geas.
However, you could try to control an enemy politician by having a Geas'd Intellect Devourer teleport into its mind and telling the Intellect Devourer what to do. If it doesn't do so, it takes damage while inside a creature. Nevertheless, I don't think this is what you're looking for.

Arcane Archer's Piercing Arrow

The Ranger subclass Arcane Archer (XGtE, p. 28) has a Piercing Arrow feature:

Piercing Arrow. You use transmutation magic to give your arrow an ethereal quality. When you use this option, you don't make an attack roll for the attack. Instead, the arrow shoots forward in a line, which is 1 foot wide and 30 feet long, before disappearing. The arrow passes harmlessly through objects, ignoring cover. Each creature in that line must make a Dexterity saving throw. On a failed save, a creature takes damage as if it were hit by the arrow, plus an extra 1d6 piercing damage. On a successful save, a target takes half as much damage.

One could argue that the sentence indicates that cover provided by creatures isn't ignored. However, that is neither RAW nor do I believe it is intended by the feature - considering that it otherwise also affects all creatures in that line, and taking the "theme" of the feature into account.

Heat Metal

This is going to be a means of last resort, and you're probably better off dropping the target to 0 HP in some other way.

That being said, you can stick a metal rod into the possessed creature's skull, and then cast heat metal on it. That way, the Intellect Devourer (which is explicitly mentioned to be inside the skull) is in contact with the rod, and affected by Heat Metal. Of course, it's gonna be impossible to stick a rod into skull mid-combat (much like you can't crack open the possessed guy's skull to hit him with a melee attack), so you could do that beforehand. However, at this point, I'd be questioning your player character's sanity if they actually did that. I don't recommend this option at all. Either way, the player of the possessed character is probably going to be super pissed, or at least his/her character probably will be once you revive him (I'm not particularly optimistic about a character's survival chances after having a glowing-hot metal rod in his head, not even mentioning his consumed brain).

Disclaimer: DO NOT TRY AT HOME.

Wish

Needless to say, Wish can accomplish anything, but that's probably not what you're interested in either.