Well, +0 K is the minimum state of entropy, absolute zero, entropy cannot decrease any further.
Adding energy to the system increases the entropy and consequently the temperature. For most (natural, I guess) systems there is no entropy saturation, meaning that the more energy you add to the system the greater the entropy — heat is indefinite, limitless, +∞ K.
However, (at least in laboratories) you can create systems with entropy saturation, meaning that once a certain point is reached, entropy becomes saturated, and adding further energy to the system no longer increases entropy. This means that adding energy to the system actually decreases entropy, the quantity of energy no longer available for work.
This saturation point, −0 K is technically the "hottest" (maximum entropy) possible state, and adding energy to this state will decrease entropy, and thus heat, towards −∞ K.
So even though it's a negative temperature, −0 K is actually hotter than +∞ K, even though +∞ K is indefinite.
It took a while for me to wrap my head around it, you really need to look at entropy rather than what is commonly thought of as heat/temperature — otherwise you'll just be left bewildered.
So, paradoxically, there is both no limit (+∞ K) and a limit (−0 K) to heat.
+0 K, adding energy increases entropy towards +∞ K
−0 K, adding energy decreases entropy towards −∞ K
Hopefully I explained that correctly

Too bad we don't have any physicists on this board
