There is a new feature coming up which gives you the opportunity to re-enable the spendlimit on a subscription.
BUT
This is not true for subscriptions you removed the spendlimit in the past!
From the next days on (Support told me rollout will be 29th of August) you have the choice to remove the spendlimit for ever or only for the current billing period.
Just chose what you like
If you removed the spendlimit permanently you will not be able to re-enable the spendlimit.
If you remove the spendlimit only for the current billing period you are able to re-enable the spendlimit.
UPDATE
Feature is now available!
And you are able to re-enable spendlimits for subscriptions you removed the spendlimit in the past.
Thank you Azure.
Gotti's Blog 