If it is still damp, you can sponge it off with warm water and a bit of Washing Up Liquid.
It is is hard and dry, meths will soften it so that you can (after a few hours) sponge it off. You may need a nylon nail-brush or similar to work it out of the pile. But don't do that much, and only in the direction of lay, never in circles, or you will frizz the pile. After brushing on the meths, cover it thoroughly with plastic wrap to prevent it drying out. You can mix a bit of WUL with the meths, it seems to help.
Kept damp for long enough, even water and WUL will soften it.
I have only used it with oil based paints and wasn't particularly impressed with it.
I recently had to remove some waterbased eggshell from some doors. I had run out of meths and so used cellulose instead. Within seconds, the 3 year old waterbased eggshell started to blister and ripple. Not sure if the cellulose will be safe on the carpet though.
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