EigenValues this will work:
SetSystemOptions["ParallelOptions" -> "ParallelThreadNumber" -> 1]
For functions which are using MKL libraries you can also use:
SetSystemOptions["MKLThreads" -> 1]
It is wrriten there that you can reset it to the defaults by
"ParallelThreadNumber" -> $ProcessorCount. But if you have cores that can be multithreaded then this number can be greater than
- Somewhere in documentation center is example with "MKLThreads" (I know it from there) but I can't find it :) I appreciate link if someone knows it :).