Most places; wether they be paintball, airsoft, or whatever; eventually end up with one of two mercy rules.
Number #1: mercys are not allowed
Number #2: mercys are allowed, but you do not have to take one.
If you tell people that they "must" except a mercy; then jag offs will try to mercy you when it's inappropriate. Like those kids running up the hill yelling "mercy, mercy, mercy, mercy!". I say the correct response to that would have been to shoot them out. Good for you for being the bigger man; but, you never want some kid to take that as a victory; lest you want him to do it again. You don't have to shoot him in the face or anything malicious. Just pop him in the love handle a couple times and say, "mercy not accepted". This way he won't be see the worth in attempting such a ridiculous thing again. Unless of course the field rules dictate that he/they are in fact allowed to do that.
Another common adative to the mercy system is this; "you cannot mercy more than one person at a time". In fact, the first mercy must be clearly accepted or unaccepted; and that situation resolved before a second is allowed to be attempted.
So if you want a straight up non conveluted way of having a mercy system; then use a variation of the following. Strictly to keep the game about sportsmanship and honor.
The purpose: the purpose of a mercy kill is to provide your opponent the option to not get shot when it's not necessary; or to quietly do away with them. Often used at close range; and only viable when they "cannot" circumvent the kill by escape or retaliation. If your opponent manages to escape or kill you instead, it's then clear you did not earn the mercy kill.
The rules: a mercy kill "must" be clearly accepted to be viable. A mercy does not have to be taken. A target player may try to escape or retaliate if they so choose. If a mercy is not immediately and clearly accepted, then it is "very" acceptable to shoot your target in lieu. Any player may never mercy more than one target at any one time. Your first target "must" clearly accept or deny the mercy; and be killed or escape before another mercy can be attempted.
Getting killed via mercy is a privilege. If you try to counteract mercy kills, it will cause your opponents to shoot you on site; regardless of distance or circumstance. Consider accepting a well earned kill; as it's more sportsmanlike for both sides.
Its Just guideline; but a common one. The best way to impliment any rule, is for it to be simple, and cut and dry.
ďREALITY IS LIKE A STONE. TO MANY ITíS HARD AND COLD, THEY CANíT HUG IT OR EAT IT, IT ONLY FRUSTERATES THEM AND DOESNíT DO THEM MUCH GOOD. TO OTHERS ITíS STRONG AND DEPENDABLE, YOU CAN BUILD WITH IT, BUILD UPON IT, OR WORK WITH IT, ALSO USE IT TO SMASH PEOPLE IN THE FACE.Ē