Dalzk it doesn't negate the effect :
1) Speed buffs are calculated from base value - Speed debuffs reduce your total speed (including your speed buffs)
So if you have basespeed 6 and speed buffs of 50% you get a total speed of 9.
If now a speed debuff of -50% is applied you end up with a speed of 4.5 which is only 75% of your base speed.
(The actual order in which the buffs/debuffs are applied is irrelevant here - doesn't change how it is calculated)
To put it mathematically :
total speed = basespeed * (1+ Sum of Speedbuffs) * (1-Sum of Debuffs)
4,5 = 6 *(1+0.50) * (1-0.50)
for 0<x<1 the result of (1+x) * (1-x) is always smaller than 1.
Disclaimer : I haven't actually tested it yet in beta3 but I think it didn't change.