You're going to need to describe this in more detail.
And - as always with repeats - why are you using them?
In general they should be avoided. Use a portal and related child records instead.
You do want to evaluate whether a Repetition is your best vehicle. I personally use them extensively and find they have many benefits outside of parent/child relationship issues.
There are ways to set all the values in one fell swoop, but they involve using Preset variables and auto-enter calculations. Executing in a script is trivial, generally fast and easier to debug. Can you explain a bit more of the context?
Short answer: no.
Long answer: you can't have a matrix, i.e. rep [ i ; j ].
Wish I could...
Addendum: the only way to set all linear repetitions of a field to something is to use lookup.
Example: if you have a field with 100 repetitions and you want to set them all to "", then you need a table with a  vector having all of them set to "", a relationship to that record and a relookup. This is the only way to empty em all in one strike.
Otherwise you have to loop yourself silly with a script, which is so baad.
I use repeating fields strictly for display. No calculations. They're generally not worth it. As mentioned above, a portal of related records is far more flexible.