General Reduction:
* Abridge: Shorten or reduce.
* Reduce: Make smaller or less in amount or degree.
* Limit: Restrict or confine within certain limits.
* Restrict: Keep within limits; confine.
* Shorten: Make shorter in duration or extent.
* Trim: Reduce something by removing an excess part.
Specific to Time or Activity:
* Cut back: Reduce or decrease something.
* Cut short: End something sooner than planned.
* Abbreviate: Shorten or reduce in length.
* Compress: Make something smaller or more concise.
More Formal:
* Diminish: Make or become smaller or less.
* Contract: Become smaller or less in size.
* Truncate: Shorten something by cutting off part of it.
The best synonym depends on the specific meaning of "curtail" in the context you're using it.