Can someone commit sin in the dream?
I mean sin of sexual immorality, if a born again God fearing christian encounter wet dreams once in a blue moon, can satan use it as legal ground to resist someone blessings, I just want to know if it's a sin to have sex in the dream once in a blue moon, like once or twice in a month.
How can someone stop indulging into any acts that can lead to sexual immorality in the dream?
How can someone resist or say no to sex in the dream,

No one is accountable for what they do in dreams. It is not a sin.

Its not a sin per se, but what the enemy is doing is to prevent you from getting your blessings through such dreams and also make you wrecked with guilt. If you were meant to get a breakthrough soon, having sex in the dream enables them to set you back especially when you have an orgasm, meaning you have released and handed whatever opportunity you were meant to get to them - hand over process. It's also another way of locking you down spiritually.
Very dangerous, but can be stopped through prayers and fasting Mark 9 v 29:Matthew 17 v 21: When next you have such dreams, go straight into 24 hours fasting, rejecting anything that has been taken from you etc and start to immerse yourself in the word of God from now. You don't have to read whole chapters, a succession of one or two scriptures and praying it out does help, but focus on how you pray it out through meditating on the whole scripture or on a word that jumps out at you. Also start to live right, look at the gifts of the Holy Spirit for guidance, am not saying you don't, but it helps to kick these things out of your spiritual life.
Then use it to deal with any situation, don't forget to start with praise and thanksgiving, then forgiveness, then you can register your request and close with thanksgiving.

Stay blessed

Thou shalt not dream