There's a rule in 3.5e that a specific rule trumps a general rule. You can find that rule in the Rules Compendium, on page 5, "Order of Rules Application."
There is a general rule that supernatural abilities cannot be dispelled, found on page 314 of the Player's Handbook:
Supernatural abilities are not subject to dispelling, disruption or spell resistance.
(This is functionally different from the SRD version of the same rule, which only mentions dispel magic. The PHB rule is the correct one.)
However, there's a specific rule that the spell haste counters and dispels slow, and the spell slow counters and dispels haste. That means that any abilities that refer to either spell, like the stone golem's Slow ability that functions "as the spell," inherit those specific rules interactions and can thus be dispelled.
Haste and slow, specifically (unlike other spell pairs like chill metal and heat metal or sympathy and antipathy), get used almost as keywords. The Slow Breath breath ability, for example, doesn't slow its targets; it slows them. It then goes on to define what that means (it's exactly like the spell slow, which is purely coincedental according to the rules, except it suppresses haste rather than dispel it). Then there's something like the Blinding Speed feat from the Epic Level Handbook, which allows one to "act as if hasted." Rules-as-written, that doesn't do anything at all. Hasted isn't a term in 3.5. Immediately, that means DM calls are necessary; can Blinding Speed be dispelled? The rule about extraordinary abilities isn't a factor, here—this is clearly a much more specific interaction—but rather whether or not the spell slow can dispel hasted as well as haste. (We're disregarding that Blinding Speed is a free action to reactivate, so dispelling it isn't very useful.)
And then there's the Swiftblade's Fortified Hustle (Ex) ability, which says
the effect becomes extraordinary rather than a continuous spell effect, and therefore cannot be dispelled by any means.
Is this just reminding us of a general rule, in which case dispel magic wouldn't work but slow's specific dispelling clause would still take precedence, or is Fortified Hustle adding the weight of specificity to a Swiftblade's extraordinary haste spell? (It's worth pointing out that having slow take effect rather simply negate haste is pretty ruinous for a swiftblade, who can recast haste as a free action.) Again, this is up to your DM.
So the interaction between Ex or Su slow and haste will vary on a case-by-case basis, but for those instances that directly inherit the text of the spell haste and the spell slow, their specific dispel interaction is a much more specific rule than the general glossary definitions that says supernatural and extraordinary abilities can't be dispelled.
