Ian Gough has been reprimanded by the Pro12 governing body for wearing a Spiderman-pattern scrum cap backing campaign group New Fathers 4 Justice.
The ex-Wales lock, 37, wore the head gear playing for Newport Gwent Dragons against Scarlets.
The cap displayed his son's initials and NF4J. Pro12 chiefs said this breached commercial rules.
"I'm pretty disappointed with the decision," said Gough who was convicted of assaulting his ex-partner in July.
"However, I do realise after it being pointed out that it conflicted with the rules concerning advertising," added the player, who won the last of his 64 Wales caps in 2010.
"I take this on board and accept their decisions," he concluded.
A Pro12 spokesperson said: "No logos, messages or advertising may be displayed on player's kit at matches save for the club logo, the kit manufacturer's logo and messages promoting club sponsors."
Earlier this year, Croyden magistrates had found Gough guilty of assaulting his former girlfriend Sophia Cahill after hearing he had pushed her into the passenger door of his campervan outside her home in London.