As suggested in the comments (thanks @Weather Vane, @Davo, @Jeff Zeitlin), there is no general rule for an antonym to "co-". It depends on the context.
In some cases, you can add more than one prefix to the word, but be aware that this can make the wording sound very technical, formal or stiff:
Co-planar can become non-coplanar
Cooperate can become non-cooperative or simply not cooperative
Many "co-" words do not have a simple antonym at all:
Coeducational becomes divided, separate or segregated.
Cohabit (from cohabitation) becomes disjoin, or simply break up or move out
There is no simple prefix like "anti" that universally does what you need.