War3:Source

have the level bank action check for race flags

race flags can be anything since its a string

after War3Source_Engine_PlayerLevelbank.sp

if(W3GetRaceMaxLevel(race)<=War3_GetLevel(client,race)){
                War3_ChatMessage(client,"%T","Your race is already maxed",client);
                return;
}

for something like

if race has flag ("nolevelbank")
{
//print some error
return;
}

Yup.
Just create a new denyable, when someone uses levelbank use W3Denyable. In the W3Deny callback you can then do your checks.
Like here

So I wanted to create a race that people can't levelbank.

I used the DN_ShowLevelbank denyable, and can block it that way.

However, if you go onto another race, then open the levelbank menu, then change race (via a chat command), you can still put levels into this race.  The denyable blocks the menu, but not the actual levels...

Any ideas for how I could block this?

Thanks.

You really should force a player's menu to close anytime they change to another menu or do something else in war3source.

Best way is to record the current player's menu handle in the war3source system, check if they are currently viewing a war3source menu, and if so... Force cancel that menu before a player activates any other command in war3source.

War3source needs that update, cause players can get away with a lot more than just level banks.

Submitted a fix for this issue to github.