There are a few ways to keep you from warping:
Warp scrambling: Using a mid-slot item, either a Warp Scrambler (-2 warp strength, 7.5KM range) or Warp Disruptor (-1 Warp strength, 20KM range)
Mobile Warp Disruptor: Commonly called a "bubble", this is a deployable device that comes in Small, Medium, and Large variants, with larger diameters for each size. It is impossible for any ship to initiate warp while inside the diameter of the Warp Disruptor field.
Bumping: Ramming a ship to prevent it from aligning to warp. This is usually only effective on slow, large ships.
Interdictors: Specialized Tech 2 ships that have a special ability to lauch Warp Disruptors, similar to small bubbles.
Ways to combat these tactics:
Warp core stabilzers: Low slot items that add +1 Warp strength for each one fitted. Effective *only* against Warp Scrambling described above.
MicroWarp Drive (MWD): Allows you to be fast enough (in some cases) to escape the range of Warp Scrambling, Bubbles, Ramming, and Interdictors, provided you aren't Stasis Webified (webbed), which cuts your maximum speed by 75% or more.
Fast alignment to warp: Smaller ships turn and accelerate more quickly than larger ones, and use of Nanofiber low slot modules can enhance this. The faster you can align and accelerate, the faster you can warp. This can prevent you from being Warp Scrambled in some cases, if you are in warp before thay can lock, and ramming attempts to a degree if they don't have time to close on you.
Electronic Counter Measures (ECM): Can be used to break the lock of a ship that is Warp Scrambling you.
Nosferatu: Energy vampire that drains the capacitor of the target ship. If you can drain the Warp Scrambler's cap, it will deactivate.
Fire back: Blow up whatever is keeping you from warping. They really get pissed when you blow up their bubbles, because they are expensive. Sometimes that alone can be satisfying enough to justify losing your ship.