I am not sure that it is exactly what you need, but the following is true:

For an alhabet with $q\geq 4$ letters and a sequence of forbidden words with lengths $n_1<n_2<\dots$ there exists an infinite word without forbidden subwords (where subword of a word W is a segment of consecutive letters in W, like "hab" is a subword of "alphabet".)

Proof. Choose $c$ like $c=2$ and prove that for the number $f(n)$ of permitted words of length $n$ (i.e. words without forbidden subwords) we have $f(n)\geq cf(n-1)$.

Induction in $n$. Base $n=1$ holds ($f(0)=1$, $f(1)\geq q-1$).

We have $f(n)\geq qf(n-1)-\sum_i f(n-n_i)$ (take any permitted word with $n-1$ letter and add arbitrary letter. If new word is not permitted, then it ends by some forbidden subword.) By induction purpose we have $f(n-i)\leq c^{1-n}f(n-1)$, thus $f(n)\geq (q-\sum c^{1-n_i})f(n-1)\geq cf(n-1)$ as desired.

Now we have arbitrarily long permitted words. It follows that there exists an infinite permitted word (proof: choose letters $x_1,x_2,\dots,x_n$ so that word $x_1\dots x_n$ has arbitrarily long permitted continuations. We may always proceed.)

Above argument works for $q=2$ and $q=3$ under stronger assumptions on $(n_i)$.