These sentences are completely different. If you say, "I did it just once", it means that you did it only one time in the past. and if you say " I'll do it just once" it means you did not do it yet but you will do it only one time in the future.

Technically, both these sentences are grammatically correct but it doesn't sound natural. To mean you'll never do it again, "I just did it once" sounds better and to mean you'll do it once and thats all, "I'll only do it once" sounds better