[Misiur]

Pretty simple:
1. Create a boolean array of all players

pawn Код:

new bool:ArmourUsed[MAX_PLAYERS char] = { false, ... };

2. When player buys armour, set his state

pawn Код:

ArmourUsed{playerid} = true;

3. When player wants to buy armour again, check if he already did it

pawn Код:

if (ArmourUsed{player}) return rejectOrSomething();

4. Reset his state on spawn or wherever you want

pawn Код:

public OnPlayerSpawn(playerid)
{
    // (...)
    ArmourUsed{playerid} = false;
    // (...)
}

And you are done!