How can I purify RNA after gel electrophoresis to remove residual acrylamide?
If you don't want acrylamide in your preparation, don't use it, as you will always have some carry-over in the solution. And you will not get rid of it completely, as it at least partly co-precipitated with the nucleic acids (it's used as a co-precipitation agent for this purpose). 

I think the best solution is to use chromatography, either size-exclusion or charged resins. See these two papers for more information:


Large-scale preparation and purification of polyacrylamide-free RNA
oligonucleotides
Rapid, non-denaturing RNA purification using weak anion-exchange fast
performance liquid chromatography