The Gnoll has hosted Neath since the club was founded in 1871
Neath Rugby are urging charity fundraisers to work with them if they want to use the club's trademarked Maltese Cross logo.
Club officials are concerned that the distinctive historic badge is being used without their approval.
They stress they do not wish to block efforts to support NHS charities during the coronavirus crisis.
The Welsh All Blacks and their supporters' club are now promoting an official fundraising shirt.
A charity match is also being planned away to local rivals Tonna when public health regulations allow.
Former Welsh champions Neath, who play in the second tier semi-pro Championship, have a trademark on the use of the Maltese Cross symbol for goods related to a sporting context in the UK, with and without the words 'Neath Rugby' and 'Neath RFC'.
They adopted the badge in the late 19th century, when they were among the top clubs in the early years of the sport in Wales.
"We have plans for charitable fundraising activity, and we're trying to do it with a coherent strategy as a club," said club CEO Matt Church. "But we've got people doing their own projects and selling merchandise without our knowledge.
"We're here as part of the community to engage in fundraising and support the NHS in this instance, but let's do it where we can buy in the merchandise and use our social media to get the most promotion.
"In the last week alone, we've seen three separate NHS fundraising jerseys created that we've had no knowledge of, all looking essentially the same, made by various apparel manufacturers, none of them (club suppliers) Samurai."
Church says the trademark warning is not intended to prevent money being generated for any good causes.
"We actively encourage and support charity fundraising, but let's do this in a coherent and professional manner," he explained.
Neath were leading lights in Welsh club rugby in the late 1980s, giving close matches to New Zealand, South Africa and Australia.
They were winners of the Welsh Premier Division in the first full season of professional rugby in 1996.
The club were co-founders of the Neath-Swansea Ospreys regional side in 2003 and also won five Welsh semi-pro titles in six years before financial problems and a dispute with the Ospreys saw them relegated to the Championship.
Neath were fifth in the table when the season came to an early end because of coronavirus, and hosted two Pro14 games in 2019-20 after repairing their links with the Ospreys.
