Are you allowed to enforce your subjective morality upon others?

[BlackSheep]

Firstly, its worth mentioning "enforcing" might of been a poor choice of words, I think people are imagining violence means of coercion. I really mean any sort of effort to change the other person's behaviour, from converstaional pressure to military pressure and everything in between.

Thank your for clarifying. I took to mean the standard meaning in terms of laws.

If you deem something is immoral, doesnt that necessarily means that some harm is been caused by it? That what makes something immoral right?

Not at all. Certainly not in an objective way. Both lying and being truthful can easily cause harm. In many cases enforcing a law causes easily as much damage if not more than not enforcing it.
Some example of morality that is often enforced (by law) that do not cause any significantly demonstrable harm.

Same sex marriage.
Offensive language.
Following a particular religion.
Pornography
etc

Being the case, that you should oppose any immorality that causes harm and also being the case that all forms of immorlaity cause some harm (from subtle (littering) to extreme (genocide)) then should'nt you oppose all immorality, including the simple sin of lying?

Only if it truly causes harm. Many people feel lying is crucial to the operation of society.