I know there are diminishing returns, but is a chain of slow able to completely immobilize someone? For example, a Nunu snowball with Rylais, a Zilean slow, a Rumble ult, a Mundo cleaver with Randuin's, and a red buff with Ashe's slow?
No this is not possible to completely stop someone, because the % don't stack. One slow would do X%, the next would do X% of the remaining percent and so on and so forth. It could continue to go down and down towards 0 but it would never reach it.
It could reach such a percentage that it could effectively stop them, say 30%. I recommend you don't do this though -- just slow the enemy group and kill them all.