There are different ways to do this, I'll just give you my version:
package BlockBuild
{
function fxDTSBrick::OnPlant(%data, %brick)
{
if(%client.blocks>=1);
{
Parent::OnPlant(%data, %brick)
%client.blocks--;
}
else
{
%brick.schedule(0,delete);
%brick.getGroup().client.centerPrint("<just:center><color:ffffff>You do not have enough blocks to place this brick",3);
}
}
};
activatePackage(BlockBuild);