can compulsive sinners, who nevertheless have the faith, they be forgiven if they keep sinning and repeanting and then carry on sinning again?
I think the nub of your question is found in the word compulsive.
Is it a matter of degree? I mean, we all sin, and probably commit the same sin (anger, pride, quick unkind words, inappropriate flippancy, &c &c) many times, but the word compulsive
has a kind of psychiatric ring to it. I knew a chap who longed to be a missionary, yet he could not break the habit of going to see pornographic films. He hated what he did, but had never found the moral strength to break the habit. (Maybe now he has: I haven't seen him for years.) Now we are not discussing whether he should become a missionary, but whether God would be merciful to him not seven times, but seventy times seven (and more). There are people, and Christians among their number, who have deeply ingrained habits which they hate and long to be free of, and yet find themselves creeping with shame and unworthiness back into the Lord's presence with words like, "Lord, I've done it again. Please forgive me," and no doubt they feel they could equal St Paul in being "the chief of sinners".
What I mean is If one believes that he's ... now Eternally Saved, goes on sinning because he knows his sins will be forgiven anyway , surely there must be some kind of limits even in Calvinism..?
Aren't you here really asking whether there are limits, not to Calvinism (which hardly matters one way or the other), but to God's mercy? I don't think such a person would "go on sinning because he knows his sins will be forgiven anyway," but would be more like the people I describe in my above paragraph. If he didn't care about the fact that he was repeatedly falling into sin, it is not likely that he would be a child of God. We are not to presume on God's mercy: thou shalt not tempt the Lord thy God (or "put the Lord to the test"). This would be akin to the temptations Satan put to our Lord in the wilderness: throw yourself down from the temple - God will protect you anyway.
May the Lord give us grace to grieve bitterly over our sins, but may he never cease to have mercy upon us. Such at least is my need before him.
What I am trying to say is that Protestants,such as Calvinists, in my opinion, would be less afraid to sin due to their belief Once Save Always Saved, than say an Orthodox who believe that he should avoid sinning at all costs because a sin is an obstacle to the path of Salvation..
We are here speculating about other people's inner anxieties or sense of security. Such a person might, as you say, feel less afraid of eternal damnation
, but sin is certainly an obstacle on the path of sanctification and growth in conformity to Christ's character, and the real "fear of the Lord" is not only fear of hell, but a deeply sensitive desire not to offend the God who has done so much for us, indeed, whose Son paid such an awful price for our salvation when he hung nailed to a Roman cross.