First a correction to your constraint:
con buyAtMostOfOdd: sum{a in ADWORDS: mod(a,2) = 1}buy[a]<=20;
Now here is a way to force 3, 5, 7:
con AtLeastOne357: sum {a in {3,5,7}} buy[a] >= 1;
And here are four alternative ways to avoid 35.
Explicit constraint:
con No35: buy[35] = 0;
Fixed variable:
fix buy[35] = 0;
Change index set declaration:
set ADWORDS = 1..100 diff {35};
Modify index set after declaration:
set ADWORDS init 1..100;
ADWORDS = ADWORDS diff {35};